Nanocrystal doped matrixes

ABSTRACT

The present invention provides matrixes doped with semiconductor nanocrystals. In certain embodiments, the semiconductor nanocrystals have a size and composition such that they absorb or emit light at particular wavelengths. The nanocrystals can comprise ligands that allow for mixing with various matrix materials, including polymers, such that a minimal portion of light is scattered by the matrixes. The matrixes of the present invention can also be utilized in refractive index matching applications. In other embodiments, semiconductor nanocrystals are embedded within matrixes to form a nanocrystal density gradient, thereby creating an effective refractive index gradient. The matrixes of the present invention can also be used as filters and antireflective coatings on optical devices and as down-converting layers. The present invention also provides processes for producing matrixes comprising semiconductor nanocrystals.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of the filing dates of U.S.Provisional Patent Application No. 60/536,962, filed Jan. 15, 2004, andU.S. Provisional Patent Application No. 60/635,784, filed Dec. 15, 2004,the disclosures of which applications are incorporated by referenceherein in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to nanocomposite matrixes, includingpolymeric layers comprising nanocrystals, and processes for preparingnanocrystal doped matrixes.

2. Background Art

High performance down-converting phosphor technologies will play aprominent role in the next generation of visible light emission,including high efficiency solid-state white lighting (SSWL). Inaddition, such technologies are also applicable to near infrared (NIR)and infrared (IR) light emitting technologies. Down-conversion fromultraviolet (UV) or blue light emitting semiconductor light emittingdiodes (LEDs) into blue, red and green wavelengths offers a fast,efficient and cost-effective path for delivering commercially attractivewhite light sources. Unfortunately, existing rare-earth activatedphosphors or halophosphates, which are currently the primary source forsolid-state down-conversion, were originally developed for use influorescent lamps and cathode ray tubes (CRTs), and therefore have anumber of critical shortfalls when it comes to the unique requirementsof SSWL. As such, while some SSWL systems are available, poor powerefficiency (<20 light lumens/watt (lm/W)), poor color rendering (ColorRendering Index (CRI)<75) and extremely high costs (>$200/kilolumen(klm)) limit this technology to niche markets such as flashlights andwalkway lighting.

Furthermore, LEDs often suffer from reduced performance as a result ofinternal reflection of photons at the chip/coating interface. Typically,LEDs are encapsulated or coated in a polymeric material (which maycomprise phosphors) to provide stability to the light-emitting chip.Currently these coatings are made by using an inorganic or organiccoating that has a very different refractive index than the basematerial (i.e., the chip), which results in a detrimental optical effectdue to the refractive index mismatch at the interface between the twomaterials. In addition, the temperature of the LED can reach in excessof 100° C. To allow for the expansion and contraction that can accompanythis temperature rise, a compliant polymeric layer (e.g., silicone) isoften placed in contact with the chip. In order to provide additionalstability to the LED, this compliant layer is often further coated witha hard shell polymer.

The resulting LED structure suffers loss of light at the chip/compliantpolymer interface due to the lower refractive index of the polymercoating in relation to the LED. However, if the refractive index of thecompliant layer is increased, even greater loss will occur due at thehigh refractive index/low refractive index interface between thecompliant polymer and the hard shell polymer due to internal reflection.

There are several critical factors which result in poor powerefficiencies when using traditional inorganic phosphors for SSWL. Theseinclude: total internal reflection at the LED-chip and phosphor layerinterface resulting in poor light extraction from the LED into thephosphor layer; poor extraction efficiency from the phosphor layer intothe surroundings due to scattering of the light generated by thephosphor particles as well as parasitic absorption by the LED chip,metal contacts and housing; broad phosphor emission in the redwavelength range resulting in unused photons emitted into the near-IR;and poor down-conversion efficiency of the phosphors themselves whenexcited in the blue wavelength range (this is a combination ofabsorption and emission efficiency). While efficiencies improve with UVexcitation, additional loss due to larger Stokes-shifted emission andlower efficiencies of LEDs in the UV versus the blue wavelength rangemakes this a less appealing solution overall.

As a result, poor efficiency drives a high effective ownership cost. Thecost is also significantly impacted from the laborious manufacturing andassembly process to construct such devices, for example theheterogeneous integration of the phosphor-layer onto the LED-chip duringpackaging (DOE and Optoelectronics Industry Development Association“Light emitting diodes (LEDs) for general illumination,” TechnologyRoadmap (2002)). Historically, blue LEDs have been used in conjunctionwith various band edge filters and phosphors to generate white light.However, many of the current filters allow photon emission from the blueend of the spectrum, thus limiting the quality of the white LED. Theperformance of the devices also suffer from poor color rendering due toa limited number of available phosphor colors and color combinationsthat can be simultaneously excited in the blue. There is a needtherefore for efficient nanocomposite filters that can be tailored tofilter out specific photon emissions in the visible (especially the blueend), ultraviolet and near infrared spectra.

While some development of organic phosphors has been made for SSWL,organic materials have several insurmountable drawbacks that make themunlikely to be a viable solution for high-efficiency SSWL. Theseinclude: rapid photodegradation leading to poor lifetime, especially inthe presence of blue and near-UV light; low absorption efficiency;optical scattering, poor refractive index matching at thechip-interface, narrow and non-overlapping absorption spectra fordifferent color phosphors making it difficult or impossible tosimultaneously excite multiple colors; and broad emission spectra. Thereexists a need therefore for polymeric layers that aid production of highquality, high intensity, white light.

The present invention fulfills these needs by providing polymericnanocomposites that function as down-converting layers, photon-filteringlayers and/or refractive index matching layers, by taking advantage ofthe ability to tailor nanocrystals to maximize their emission,absorption and refractive index properties.

BRIEF SUMMARY OF THE INVENTION

The present invention provides matrix materials doped with nanocrystalsthat have specific emission and/or absorption characteristics and alsoallow for specific tailoring of refractive indexes of thenanocomposites.

In one embodiment, the present invention provides polymeric layerscomprising a polymer and semiconductor nanocrystals embedded within thepolymer, wherein the nanocrystals have a size and a composition suchthat they absorb visible, ultraviolet, near-infrared and/or infraredlight, and wherein the polymeric layers scatter a minimal portion oflight that enters the layers. In certain embodiments, the polymer issilicone. The polymeric layers of the present invention can be used tocoat optical devices (e.g., refractive lenses or reflective elements) orcan be used to encapsulate active devices, such as a light emittingdiodes (LEDs). Suitably, the polymeric layers of the present inventionthat absorb visible light will absorb red light, blue light and/or greenlight.

The nanocrystals utilized throughout the embodiments of the presentinvention will suitably be between about 1-10 nm in size, about 1-4 nmin size or about 1-3 nm in size and can further comprisemiscibility-enhancing ligands attached to their surface to aid in mixingwith the polymers. The polymeric layers of the present invention canhave any effective refractive index between that of the pure polymer andthe pure nanocrystals, and will suitably have an effective refractiveindex greater than about 1.5 and in certain embodiments about 1.8. Incertain embodiments, the polymeric layers of the present invention willbe greater than about 0.5 mm in thickness

In another embodiment, the present invention provides polymeric layerscomprising a polymer and semiconductor nanocrystals embedded within thepolymer, wherein the polymeric layer has an effective refractive indexgreater than the polymer alone, and wherein the polymeric layer scattersa minimal portion of light that enters the polymeric layer. Suitably,the polymeric layers will scatter less than about 50%, less than about20% or less than about 15% of light that enters the polymeric layers. Insuitable embodiments, the nanocrystals will be ZnS nanocrystals and thepolymeric layers will be greater than about 0.5 mm in thickness.

In another embodiment, the present invention provides polymeric layersthat encapsulate an active device (e.g., an LED) that has an effectiverefractive index, n₁. The layer comprises a polymer and semiconductornanocrystals embedded within the polymer. The layer has an innerboundary in contact with the active device and an outer boundary incontact with a medium having an effective refractive index, n₂, whereinthe layer has an effective refractive index less than or equal to n₁ atthe inner boundary and an effective refractive index greater than orequal to n₂ at the outer boundary. In certain embodiments, effectiverefractive index n₁ will be greater than n₂, suitably greater than about1.5, and in certain embodiments about 1.8. In certain such embodiments,the layer will have a nanocrystal density gradient, being highest at theinner boundary and lowest at the outer boundary. Suitably thisnanocrystal density gradient will be linear throughout the polymericlayer.

The present invention also provides processes for preparing polymericlayers, comprising mixing semiconductor nanocrystals at a first densitywith a solvent and a polymer to form a first mixture, coating asubstrate material with the first mixture and evaporating the solvent toform the polymeric layer, wherein the polymeric layer has an effectiverefractive index of n₁.

The processes of the present invention can be used to prepare polymericlayers for coating active devices (e.g., LEDs), or optical devices(e.g., refractive lenses or reflective elements). The processes of thepresent invention can utilize nanocrystals which further comprisemiscibility-enhancing ligands attached to their surface.

In suitable embodiments, the processes of the present invention canfurther comprise mixing semiconductor nanocrystals at a second densitywith a solvent and a polymer to form a second mixture, coating thesubstrate material with the second mixture and evaporating the solventto form a second polymeric layer, wherein the second polymeric layer hasan effective refractive index of n₂. In other embodiments, the processescan further comprise repeating these steps with a third through i^(th)density of semiconductor nanocrystals to produce third through i^(th)polymeric layers, wherein the third through i^(th) polymeric layers haveeffective refractive indices, n₃ through n_(i), respectively. In certainsuch embodiments, the effective refractive index n₁ will be greater thann₂ and the effective refractive index of the i^(th) polymeric layer willbe less than the effective refractive index of any other polymericlayer. The processes of the present invention can further comprisecentrifuging the first mixture of semiconductor nanocrystals, solventand polymer, to form a nanocrystal density gradient within the mixtureprior to coating the substrate material.

In suitable embodiments of the processes of the present invention, thecoating can be via spin coating or screen printing. As discussedthroughout, the nanocrystals used in the processes of the presentinvention can have a size and a composition such that they absorb lightat a particular wavelength. In other embodiments, the nanocrystals canbe tuned so as to emit light at a particular wavelength. In otherembodiments, the process of the present invention can utilizesemiconductor nanocrystals that comprise two or more different sizes orcompositions and therefore can have different properties. The polymericlayers produced by the processes of the present invention will suitablybe greater than about 0.5 mm in thickness.

In another embodiment, the present invention provides solid state whitelighting devices comprising a power efficiency greater than 25 lm/W,suitably greater than 50 lm/W, greater than 100 lm/W, greater than 150lm/W, or greater than 200 lm/W.

In other embodiments, the solid state white lighting devices comprise adown converting nanocomposite that comprises two or more semiconductornanocrystals tuned to emit light at one or more selected wavelengths.The solid state white lighting devices of the present invention willsuitably provide a CRI of greater than about 80. In still otherembodiments, the solid state white lighting devices comprise a matrixcoupled to the two or more semiconductor nanocrystals via one or morechemical moieties.

Another embodiment the present invention provides down convertingnanocomposite devices, comprising two or more semiconductor nanocrystalphosphors of two or more sizes, the nanocrystal phosphors tuned to emitlight at one or more selected wavelengths, and providing a CRI ofgreater than about 80; a matrix with a high index of refraction, low UVdegradation and/or matched thermal expansion; and a chemical structurecoupling the matrix to the nanocrystal phosphors. Suitably, the two ormore semiconductor nanocrystal phosphors will comprise a core-shellstructure, wherein a shell (e.g., ZnS) provides a type I band gap withrespect to a core. The core-shell nanocrystals of the present inventionwill suitably have a quantum efficiency of about 10% to about 90%.

In further embodiments of the present invention, the two or moresemiconductor nanocrystal phosphors are color matched and the matrix cancomprise TiO₂. In yet further embodiments, the nanocomposite can belayered on an LED substrate which comprises sapphire or SiC. Suitably,the matrix will be a compliant layer that can withstand the thermalexpansion that results when the LED heats up, and suitably will besilicone.

In another embodiment, the present invention provides polymeric layers,comprising a polymer; and semiconductor nanocrystals embedded within thepolymer, wherein the nanocrystals have miscibility-enhancing ligandsconjugated to their surface, and wherein the ligands comprise an alkanechain of between 6 and 18 carbons in length. In suitable embodiments,the ligands can comprise an alkane chain of between 12 and 18 carbons inlength. The polymer will suitably be silicone, and the semiconductornanocrystals will suitably have a size between about 1-10 nm, and incertain embodiments will be ZnS nanocrystals. In certain embodiments,the polymeric layers will scatter a minimal portion of light that enterssaid polymeric layer. Suitably, the layer will be greater than about 0.5mm in thickness.

Additional features and advantages of the invention will be set forth inthe description that follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. Theadvantages of the invention will be realized and attained by thestructure and particularly pointed out in the written description andclaims hereof as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form a partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles of theinvention and to enable a person skilled in the pertinent art to makeand use the invention.

FIG. 1 shows absorption and emission spectra for various nanocrystalradii showing continuous tailoring of the emission and absorptionwavelengths.

FIG. 2 shows a comparison of traditional thick phosphors integratedduring packaging, and a nanocomposite down-converting layer integratedprior to dicing, according to one embodiment of the present invention.

FIG. 3 shows the elimination of wasted light at the edges of the visiblespectrum by using phosphor nanocrystals, compared to traditionalphosphor edge losses.

FIG. 4 shows the normalized intensity generated by mixing a continuum ofnanocrystal sizes, creating broad-band white light.

FIG. 5 shows a three color emitting LED in accordance with oneembodiment of the present invention.

FIG. 6 is a cross-sectional view of a polymeric layer in accordance withone embodiment of the present invention.

FIG. 7 is a cross-sectional view of a polymeric layer having ananocrystal density gradient in accordance with one embodiment of thepresent invention.

FIG. 8 is a cross-sectional view of an optical device with a polymericlayer coating the device in accordance with one embodiment of thepresent invention.

FIG. 9 is a plot showing the effective refractive index of variousmatrixes versus volume loading ratio of ZnS nanocrystals.

FIG. 10 is a plot showing the effective refractive index of a siliconenanocomposite comprising ZnS nanocrystals as a function of wavelength.

FIG. 11 is a cross-sectional view of a light emitting diode encapsulatedwithin a polymeric layer in accordance with one embodiment of thepresent invention.

FIG. 12 is a cross-sectional view of a light emitting diode encapsulatedwithin a polymeric layer having a nanocrystal density gradient inaccordance with one embodiment of the present invention.

FIG. 13 is a traditional LED chip—silicon cap assembly.

FIG. 14 is a nanocomposite—LED chip assembly in accordance with oneembodiment of the present invention.

FIG. 15 is a nanocomposite—LED chip assembly in accordance with oneembodiment of the present invention.

FIG. 16 is a plot of percent transmittance for a silicone nanocompositecomprising ZnS nanocrystals as a function of nanocrystals size.

FIG. 17 is a plot of percent transmission for a silicone nanocompositecomprising ZnS nanocrystals as a function of wavelength.

FIG. 18 shows a representation of a 3-part ligand, with a tail-group, ahead group and a middle/body-group.

FIG. 19 is an example ligand that can be conjugated to the nanocrystalsof the present invention.

FIGS. 20 a-20 n show examples, chemical synthesis, and NMRcharacterization of several example ligands in accordance with thepresent invention.

FIG. 21 is a flowchart depicting processes for preparing polymericlayers in accordance with the present invention.

FIG. 22 is a cross-sectional view of a polymeric layer comprisingindividual layers each with a different nanocrystal density gradientaccording to one embodiment of the present invention.

FIG. 23 shows an X-Ray diffraction analysis of ZnS nanocrystals.

FIG. 24 shows Transmission Electron Micrographs of ZnS nanocrystals.

The present invention will now be described with reference to theaccompanying drawings. In the drawings, like reference numbers indicateidentical or functionally similar elements.

DETAILED DESCRIPTION OF THE INVENTION

It should be appreciated that the particular implementations shown anddescribed herein are examples of the invention and are not intended tootherwise limit the scope of the present invention in any way. Indeed,for the sake of brevity, conventional electronics, manufacturing,semiconductor devices, and nanocrystal, nanowire (NW), nanorod,nanotube, and nanoribbon technologies and other functional aspects ofthe systems (and components of the individual operating components ofthe systems) may not be described in detail herein. It should further beappreciated that the manufacturing techniques described herein can beused to create any semiconductor device type, and other electroniccomponent types. Further, the techniques would be suitable forapplications in electrical systems, optical systems, consumerelectronics, industrial or military electronics, wireless systems, spaceapplications, or any other application.

The present invention provides various polymeric nanocompositescomprising polymeric materials with embedded nanocrystals. The variousproperties of the nanocrystals, including their absorption properties,emission properties and refractive index properties, are utilized tocreate nanocomposites that can be tailored and adjusted for variousapplications. In one embodiment, the present invention providesapplications of semiconductor nanocrystals that utilize their emissionproperties in down-conversion applications. In another embodiment, thepresent invention combines two, non-electronically active properties ofthe same nanocrystals, by using the high absorption coefficient andrelatively sharp band edge of the nanocrystals to filter light as acutoff filter. In another embodiment, the high refractive index ofnanocrystals can also be used when mixed into low refractive indexmaterials to create substantially transparent nanocomposites witheffective refractive indexes matched to the substrates they are coating.In further embodiments, the refractive index of the nanocomposite can bematched to a second, further encapsulating material. The presentinvention also provides for nanocomposites that combine two or more ofthese various properties in different configurations into the samenanocomposite.

As used herein, the term “nanocrystal” refers to nanostructures that aresubstantially monocrystalline. A nanocrystal has at least one region orcharacteristic dimension with a dimension of less than about 500 nm, anddown to on the order of less than about 1 nm. As used herein, whenreferring to any numerical value, “about” means a value of ±10% of thestated value (e.g. “about 100 nm encompasses a range of sizes from 90 nmto 110 nm, inclusive). The terms “nanocrystal,” “nanodot,” “dot” and“quantum dot” are readily understood by the ordinarily skilled artisanto represent like structures and are used herein interchangeably. Thepresent invention also encompasses the use of polycrystalline oramorphous nanocrystals.

Typically, the region of characteristic dimension will be along thesmallest axis of the structure. Nanocrystals can be substantiallyhomogenous in material properties, or in certain embodiments, can beheterogeneous. The optical properties of nanocrystals can be determinedby their particle size, chemical or surface composition. The ability totailor the nanocrystal size in the range between about 1 nm and about 15nm enables photoemission coverage in the entire optical spectrum tooffer great versatility in color rendering. Particle encapsulationoffers robustness against chemical and UV deteriorating agents.

Nanocrystals for use in the present invention can be produced using anymethod known to those skilled in the art. Suitable methods are disclosedin U.S. patent application Ser. No. 10/796,832, filed Mar. 10, 2004,U.S. patent application Ser. No. 10/656,910, filed Sep. 4, 2003 and U.S.Provisional Patent Application No. 60/578,236, filed Jun. 8, 2004, thedisclosures of each of which are incorporated by reference herein intheir entireties. The nanocrystals for use in the present invention canbe produced from any suitable material, suitably an inorganic material,and more suitably an inorganic conductive or semiconductive material.Suitable semiconductor materials include those disclosed in U.S. patentapplication Ser. No. 10/796,832 and include any type of semiconductor,including group II-VI, group III-V, group IV-VI and group IVsemiconductors. Suitable semiconductor materials include, but are notlimited to, Si, Ge, Sn, Se, Te, B, C (including diamond), P, BN, BP,BAs, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb,AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, ZnO, ZnS, ZnSe, ZnTe, CdS,CdSe, CdTe, HgS, HgSe, HgTe, BeS, BeSe, BeTe, MgS, MgSe, GeS, GeSe,GeTe, SnS, SnSe, SnTe, PbO, PbS, PbSe, PbTe, CuF, CuCl, CuBr, CuI,Si₃N₄, Ge₃N₄, Al₂O₃, (Al, Ga, In)₂ (S, Se, Te)₃, Al₂CO, and anappropriate combination of two or more such semiconductors.

In certain aspects, the semiconductor nanocrystals may comprise a dopantfrom the group consisting of: a p-type dopant or an n-type dopant. Thenanocrystals useful in the present invention can also comprise II-VI orIII-V semiconductors. Examples of II-VI or III-V semiconductornanocrystals include any combination of an element from Group II, suchas Zn, Cd and Hg, with any element from Group VI, such as S, Se, Te, Po,of the Periodic Table; and any combination of an element from Group III,such as B, Al, Ga, In, and Tl, with any element from Group V, such as N,P, As, Sb and Bi, of the Periodic Table.

The nanocrystals useful in the present invention can also furthercomprise ligands conjugated, cooperated, associated or attached to theirsurface as described throughout. Suitable ligands include any groupknown to those skilled in the art, including those disclosed in U.S.patent application Ser. No. 10/656,910 and U.S. Provisional PatentApplication No. 60/578,236. Use of such ligands can enhance the abilityof the nanocrystals to incorporate into various solvents and matrixes,including polymers. Increasing the miscibility (i.e., the ability to bemixed without separation) of the nanocrystals in various solvents andmatrixes allows them to be distributed throughout a polymericcomposition such that the nanocrystals do not aggregate together andtherefore do not scatter light. Such ligands are described as“miscibility-enhancing” ligands herein.

As used herein, the term nanocomposite refers to matrix materialscomprising nanocrystals distributed or embedded therein. Suitable matrixmaterials can be any material known to the ordinarily skilled artisan,including polymeric materials, organic and inorganic oxides.Nanocomposites of the present invention can be layers, encapsulants,coatings or films as described herein. It should be understood that inembodiments of the present invention where reference is made to a layer,polymeric layer, matrix, or nanocomposite, these terms are usedinterchangeably, and the embodiment so described is not limited to anyone type of nanocomposite, but encompasses any matrix material or layerdescribed herein or known in the art.

I. Down-Converting Nanocomposites

In order to become competitive with traditional lighting fromfluorescent and incandescent lights, significant improvements must bemade in solid-state white lighting (SSWL). Improvements not just in thequantum efficiency of the phosphors, but in all aspects of thedown-conversion system that relate to efficiency, color-rendering andoverall system cost are needed. In one embodiment, the present inventionprovides a complete down-conversion system based on engineerednanocomposite materials for use with currently available blue LEDexcitation sources that dramatically improve the overall cost,performance and efficiency of SSWL. The down-converting nanocompositesof the present invention utilize the emission properties of nanocrystalsthat are tailored to absorb light of a particular wavelength and thenemit at a second wavelength, thereby providing enhanced performance andefficiency of active sources (e.g., LEDs). As such, the nanocrystalsutilized in the down-converting applications of the present inventionwill be constructed and tailored so as to be highly emitting. In oneembodiment, this system produces SSWL that exceeds performance of thebest traditional fluorescent and incandescent bulbs, with colorrendering of greater than 80 and power efficiency of greater than 200lm/W, at a cost of less than one U.S. dollar/klm.

Performance Characteristics of SSWL Devices

To evaluate the performance characteristics of solid-state whitelighting (SSWL) devices, three primary attributes are commonly used: (1)luminous efficiency, (2) Correlated Color Temperature (CCT) and (3)Color Rendering Index (CRI). DOE and Optoelectronics IndustryDevelopment Association “Light emitting diodes (LEDs) for generalillumination,” Technology Roadmap (2002).

The luminous efficiency (measured in lm/W) is the efficiency of theconversion from electrical power (W) to optical power (W), combined withthe efficiency of the conversion from optical power (W) to the luminousflux. The luminous efficiency is influenced by a number of factors, andcan, in general terms, be written as a contribution of several separateefficiencies:E _(luminous)=η_(wp)×η_(lum)×η_(ss)×η_(IQE)×η_(package)(E _(os) , E_(pa) , E _(TIR) , E _(exp))×where η_(wp) is the wall plug efficiency, η_(lum) is the photopicefficiency/response of the human eye, η_(ss) is the stokes shiftefficiency from converting a blue photon to a longer wavelength photon,η_(IQE) is the internal quantum efficiency of the phosphor, andη_(package) is overall package efficiency and accounts for losses inlight extraction efficiency from optical scattering (E_(os)), parasiticscattering (E_(ps)), total internal reflection (E_(TIR)), externalpackaging like the lead frame and submount (E_(exp)), etc.

CCT or correlated color temperature refers to the human eye property ofbeing optimally adapted to the sunlight spectral content. The relativeintensities of the blue (B), red (R) and green (G) colors, for thedesired white color, referred to as chromaticity coordinates, optimallyreproduce those in the visible sunlight, which corresponds to ablackbody spectral distribution of 6000 Kelvin (K). For optimumillumination the chromaticity coordinates for R, G and B must fall nearthe black body radiation, for temperatures between 2000° C. and 8000° C.Higher or lower than “optimum” temperatures register to the eye as too“cold” or too “warm” color hues.

Color rendering has to do with the appearance of various object colorsunder a given source illumination, compared to that from a referencesource. A collection of 14 sample colors of different saturation iscustomarily used for the color rendering index (CRI), which provides aquantitative measure on a scale of 1 to 100. Two sources of similarcolor temperature may produce widely varying CRIs. Low CRIs make colorsunacceptable for illumination, while high CRI (>80) are acceptable forgeneral illumination purposes.

Procedure for Providing an Optimized White Light Emitting Device

In one embodiment, the present invention provides processes comprising:

(1) A simulation model can be used to determine optimized nanocrystalmixtures for CRI, CCT, and luminous efficiency with targets of CRI>80,CCT about 4,000 K and efficiency of 200 lm/W.

(2) Nanocrystals and nanocrystal component mixtures are synthesized withemission peak widths, peak maximums, and intensity ratios determined bysimulation.

(3) A controlled nanocrystal phosphor nanocomposite is developed,including: (a) a surface ligand capable of achieving high (about 20% ormore) loading density in the selected composite is produced; (b) aligand exchange process to incorporate a 3-part ligand onto thenanocrystal is performed; (c) a homogeneous, non-phase separated TiO₂nanocomposite with nanocrystal loading densities up to 20% by volume isproduced; (d) quantum yield (QY) dependence on nanocrystal loadingdensity in the nanocomposite is determined; (e) an index of refractiondependence on loading density in the nanocomposite and index-matching ofthe nanocomposite to blue LED substrate (e.g., sapphire and/or SiC) isdetermined; and (f) a relationship of loading density and film thicknessto optimize refractive index matching and nanocomposite optical densityis determined.

Simulations for Determining Optimum Nanocrystal Component Mixture forHigh Color Rendering, Color Temperature and High Efficiency

In order to predict and maximize CRI, CTT and luminous efficiency ofnanocrystal mixtures, a dynamic and robust simulation model is used. Asuper-convergent, random-search, parameter optimization algorithm isused to find the maximum performance point, subject to the imposedconstraints. The model allows calculation of these performancecharacteristics based on actual experimental colorimetric and opticalcharacteristics of nanophosphor components and mixtures. In turn, thismodel is used to aid the design and fabrication of optimal nanocompositeSSWL devices.

The simulation program incorporates an algorithm to determine theoptimum spectral emission characteristics of nanocrystal componentmixtures for simultaneous maximization of color rendering, colortemperature and overall efficacy for production of white light. Theapproach provides a super-convergent, random search, optimizationalgorithm in the phosphor parameter space. The program seeks acombination of emission wavelengths that simultaneously maximizesluminous efficacy, color rendering (CRI) and color temperature (CCT),subject to the white-light chromaticity constrains calculated usingstandard CIE (Commission Internationale de l'Eclairage). The measurednanocrystal quantum efficiency, peak wavelength and emission spectralwidth are input parameters. Performance boundaries, as for example,efficacy no less than 90%, or CRI>90, can also be applied forflexibility in the design. The number of required wavelengths (i.e.,nanocrystal sizes) is a variable that allows a determination oftrade-offs between performance and manufacturing cost.

A validation procedure with iteration cycles is adopted, wherebymixtures of nanocrystal components, of size, composition peak maximum,peak widths, mixture abundance, and internal quantum efficiencypredicted by the simulation are manufactured. The resulting values ofCRI and CCT are determined experimentally and compared with thepredictions and adjustments are made as appropriate. The luminousefficiency is determined based on optical parameters including stokeshift efficiency, internal quantum efficiency and photopic response.

The output of this procedure is the optimum number of emission colors,the precise center wavelengths of each color, the precise spectral widthof each color and the exact relative intensity of each and thecorresponding concentration based on excitation by, for example, aselected blue LED (about 460 nm).

The simulations described throughout can determine suitable emissioncharacteristics for the nanocrystal. In addition, it is useful to (1)synthesize the materials with the prescribed spectral characteristicsand (2) use the materials to validate the model. To achieve thisobjective, available solution phase synthetic techniques are used tofabricate core/shell nanocrystal phosphors and characterize mixtures asdetermined by the theoretical model.

Based on current methods, nanocrystal batches are fabricated withspectral characteristics generated by the theoretical model. Eachdistinct wavelength is synthesized separately and combined to producethe final mixture. Specific attention is paid to the center wavelengthand the peak-width of each sample. In particular, narrow emission in thered avoids efficiency loss in the IR. In order to accomplish this, asolution-phase mixture of nanocrystals is produced and characterizedthat has the appropriate composition to produce white light with CRI andCTT, matching that of the theoretical model when illuminated with blueexcitation and total down-conversion efficiency comparable to thatpredicted by the model, assuming zero loss to other mechanisms in theprocess. These measurements can be made in the solution-phase using astandard visible fluorometer and fluorescence standards with excitationmatching the blue-LED.

Nanocrystal Phosphors

While any method known to the ordinarily skilled artisan can be used tocreate nanocrystal phosphors, suitably, a solution-phase colloidalmethod for controlled growth of inorganic nanomaterial phosphors isused. See Alivisatos, A. P., “Semiconductor clusters, nanocrystals, andquantum dots,” Science 271:933 (1996); X. Peng, M. Schlamp, A.Kadavanich, A. P. Alivisatos, “Epitaxial growth of highly luminescentCdSe/CdS Core/Shell nanocrystals with photostability and electronicaccessibility,” J. Am. Chem. Soc. 30:7019-7029 (1997); and C. B. Murray,D. J. Norris, M. G. Bawendi, “Synthesis and characterization of nearlymonodisperse CdE (E=sulfur, selenium, tellurium) semiconductornanocrystallites,” J. Am. Chem. Soc. 115:8706 (1993). This manufacturingprocess technology leverages low cost processability without the needfor clean rooms and expensive manufacturing equipment. In these methods,metal precursors that undergo pyrolysis at high temperature are rapidlyinjected into a hot solution of organic surfactant molecules. Theseprecursors break apart at elevated temperatures and react to nucleatenanocrystals. After this initial nucleation phase, a growth phase beginsby the addition of monomers to the growing crystal. The result isfreestanding crystalline nanoparticles in solution that have an organicsurfactant molecule coating their surface.

Utilizing this approach, synthesis occurs as an initial nucleation eventthat takes place over seconds, followed by crystal growth at elevatedtemperature for several minutes. Parameters such as the temperature,types of surfactants present, precursor materials, and ratios ofsurfactants to monomers can be modified so as to change the nature andprogress of the reaction. The temperature controls the structural phaseof the nucleation event, rate of decomposition of precursors, and rateof growth. The organic surfactant molecules mediate both solubility andcontrol of the nanocrystal shape. The ratio of surfactants to monomer,surfactants to each other, monomers to each other, and the individualconcentrations of monomers strongly influence the kinetics of growth.

In suitable embodiments, CdSe is used as the nanocrystal material, inone example, for visible light down-conversion, due to the relativematurity of the synthesis of this material. Due to the use of a genericsurface chemistry, it is also possible to substitutenon-cadmium-containing nanocrystals.

Core/Shell Nanocrystals

In semiconductor nanocrystals, photo-induced emission arises from theband edge states of the nanocrystal. The band-edge emission fromnanocrystals competes with radiative and non-radiative decay channelsoriginating from surface electronic states. X. Peng, et al., J. Am.Chem. Soc. 30:7019-7029 (1997). As a result, the presence of surfacedefects such as dangling bonds provide non-radiative recombinationcenters and contribute to lowered emission efficiency. An efficient andpermanent method to passivate and remove the surface trap states is toepitaxially grow an inorganic shell material on the surface of thenanocrystal. X. Peng, et al., J. Am. Chem. Soc. 30:7019-7029 (1997). Theshell material can be chosen such that the electronic levels are type Iwith respect to the core material (e.g., with a larger bandgap toprovide a potential step localizing the electron and hole to the core).As a result, the probability of non-radiative recombination can bereduced.

Core-shell structures are obtained by adding organometallic precursorscontaining the shell materials to a reaction mixture containing the corenanocrystal. In this case, rather than a nucleation-event followed bygrowth, the cores act as the nuclei, and the shells grow from theirsurface. The temperature of the reaction is kept low to favor theaddition of shell material monomers to the core surface, whilepreventing independent nucleation of nanocrystals of the shellmaterials. Surfactants in the reaction mixture are present to direct thecontrolled growth of shell material and ensure solubility. A uniform andepitaxially grown shell is obtained when there is a low lattice mismatchbetween the two materials. Additionally, the spherical shape acts tominimize interfacial strain energy from the large radius of curvature,thereby preventing the formation of dislocations that could degrade theoptical properties of the nanocrystal system.

In suitable embodiments, ZnS can be used as the shell material usingknown synthetic processes, resulting in a high-quality emission. Asabove, if necessary, this material can be easily substituted if the corematerial is modified.

Optical Properties of Core-shell Nanocrystals

Due to the finite size of the core-shell nanocrystals, they displayunique optical properties compared to their bulk counterparts. Theemission spectrum is defined by a single Gaussian peak, which arisesfrom the band-edge luminescence. The emission peak location isdetermined by the core particle size as a direct result of quantumconfinement effects. For instance, by adjusting the particle diameter inthe range of 2 nm and 15 nm, the emission can be precisely tuned overthe entire visible spectrum (FIG. 1). FIG. 1 represents the absorptionand emission peaks for nanocrystals of increasing size (2 nm to 15 nm).The initial peak (lower wavelength) indicates the absorption wavelengthand the later peak (higher wavelength) the emission wavelength in nm.With increasing size of the nanocrystals, the absorption and emissionpeak wavelengths shift from about 450 nm to about 700 nm, and can betuned over this range. The vertical shaded bars on FIG. 1 indicatevisible light wavelengths in the blue 100, green 102 and red 104 ranges.

The width of the emission peak is determined by the size distribution ofthe sample. Peak widths down to 20 nm full width at half maximum (FWHM)can be achieved. Conversely, the absorption spectrum of nanocrystals isvery broad and intense, as typical of the bulk material, which ischaracteristically different than organic phosphors. Absorptioncoefficients are in excess of 55,000/cm (in the blue range of thespectrum) over the entire range of crystal sizes. In addition,core-shell nanocrystals can be made with quantum efficiencies as high as90% (this does not take into account energy loss due to Stokes shift,but is simply the ratio of photons-in to photons-out).

In one embodiment, the present invention provides an engineerabledown-converting system (see FIG. 2). Systems according to the presentinvention can comprise a nanocomposite down-converting layer that can becoated directly onto an LED wafer prior to dicing and packaging,eliminating the need for heterogeneous integration of the phosphor layerduring packaging. The nanocomposite down-converting layer is suitablyengineered from three components, including: (1) Semiconductornanocrystal phosphors of one or more, suitably two or more, sizes tunedto emit at the required wavelength(s) and with the required spectralcharacteristics to optimize the color rendering index (CRI) and powerconversion efficiency in the final device; (2) A host matrix (e.g., apolymer) selected for high index of refraction (generally about 1.5 orgreater), low UV degradation and matched thermal expansion to the LEDchip; and, (3) A unique nanocrystal surface chemistry that acts as theinterface between the nanocrystals and the host matrix, allowing eachelement to be independently selected and tailored without impacting theother component. As shown in FIG. 2, such a down-convertingnanocomposite phosphor layer 208 will take the place of phosphor 200 andphosphor encapsulant layer 206.

By independently selecting and tuning each of these three components, itis possible to simultaneously: (1) engineer a specific compositeemission spectrum that can be tailored to optimize between CRI anddown-conversion efficiency; (2) refractive index match the compositelayer to the LED chip to reduce light-extraction losses prior to downconversion; (3) reduce scattering in the down-conversion layer, therebyminimizing light-extraction losses from the phosphor layer; (4) producedown-conversion with a quantum efficiency greater than about 20% (e.g.,40%, 60%, 80%, 100%) at any wavelength with simultaneous and efficientabsorption of light (about 300 nm) (depending upon size and compositionof the nanoparticles); and (5) minimize loss of efficiency due tophotons emitted into the near-infra-red (near-IR) through the use ofextremely sharp emission spectra in red light wavelengths (about 20 nmFWHM). This approach makes it possible to achieve overallpower-conversion efficiencies greater than 200 lm/W, with CRI greaterthan 80, and overall chip brightness of greater than 100 Watts/chip at acost of less than one U.S. dollar/klm.

Nanocomposite System Features Benefit to SSWL High quantum efficiency Noloss of photons upon down (as high as 90%) conversion resulting in2-fold increase in overall power- conversion efficiency over traditionalphosphors. Continuous, tunable emission The emission peak wavelength andspectrum width can be precisely tuned so mixtures of different sizednanocrystals can be formed with precise emission characteristics toachieve maximum emission efficiency, CRI, CTT. Narrow and Sharp EmissionSharp emission allows tailoring of emitted light at wavelengths wherethe luminous efficiency of photopic vision of the eye is high. Highphoto- and chemical Nanocrystals are not susceptible stability tobleaching effects and environmental sensitivities (UV, moisture, oxygen)as traditional organic phosphors offering long operating lifetimes.Mixtures of nanocrystals in Mixtures of nanocrystals can be host matrixembedded in a host matrix of virtually any material at high- loadingdensities (e.g., 20% by volume) with precise control over relativeconcentration ratios through modification of surface chemistry.Non-scattering composites Due to the small particle size and capabilityto make homogenous dispersion of the nanocrystals, optical scattering aswell as parasitic absorption can be minimized or eliminated to improvelight extraction efficiency and hence the device luminous efficiency.Tunable refractive index By selecting the proper host matrix materialand tailoring the loading density, the index of refraction of thenanocomposite layer can be precisely tuned from about 1.5 to about 2.5to minimize or even eliminate total internal reflection at theLED-nanocomposite interface, potentially increasing overall powerconversion efficiency. Loading density and thickness can be traded-offto simultaneously optimize index of refraction and optical density ofthe composite layer while maintaining film thicknesses. High absorptioncoefficients At a high loading density, the (as high as 55,000/cm)nanocomposite down-conversion layer can be on the order of a singlemicron in thickness. This allows direct incorporation at the wafer-levelusing traditional thin film processing, dramatically reducing overallmanufacturing costs for SSWL relative to thick- film phosphor layersthat are incorporated at the package level.

FIG. 3 illustrates the emission range of the down-convertingnanocomposites of the present invention, in the red region of a 2-colorphosphor mix, compared to that resulting from traditional inorganicphosphors for white. Emission peaks 302 and 304 represent the emissionspectra of a 2-color phosphor mix according to one embodiment of thepresent invention. Spectrum 306 represents the emission spectrum oftraditional inorganic phosphors. Not only does the narrow emissionprevent photon waste at the edges of the visible spectrum by the eye,but it also allows a superior optimization of color rendering index andpower conversion efficiency. Wasted light region 308 demonstrates lightemitted from traditional inorganic phosphors at the edges of the visiblespectrum that is cut out by using the sharp emission peak 304.

FIG. 4 illustrates the concept of fine-tuning the emission by using morethan three emission colors, each with a specific, narrow, emission peak,to generate an overall emission spectrum with a superior color renderingindex that can be as high as 100 for any color temperature. Between thetwo extremes of extremely broad emission and extremely narrow emission,however, is a balance between efficiency and CRI. The exact number ofcolors, center wavelengths, relative concentrations and spectral widthscan be determined theoretically to optimize both parameterssimultaneously.

By using standard thin-film and lithographic processing techniques, asshown in FIG. 5, green 500 and red 502 down-conversion layers can bepatterned across LED chips 204 prior to dicing. This allows low-costfabrication of 3-color emitting LEDs integrated into a single die, suchthat a single chip can be used to dynamically tune emission of the LEDfrom monochromatic to white for any color temperature. As such, thepresent invention provides formation of an integrated chip-level 3-colormixing-based SSWL for all lighting applications, at a cost pointcompetitive with traditional lighting, but with far superior efficiency,performance and color engineering capability.

Suitable matrixes for use in all embodiments of the present inventioninclude polymers and organic and inorganic oxides. Suitable polymers foruse in the matrixes of the present invention include any polymer knownto the ordinarily skilled artisan that can be used for such a purpose.In suitable embodiments, the polymer will be substantially translucentor substantially transparent. Such polymers include, but are not limitedto, poly(vinyl butyral):poly(vinyl acetate), silicone and derivatives ofsilicone, including, but not limited to, polyphenylmethylsiloxane,polyphenylalkylsiloxane, polydiphenylsiloxane, polydialkylsiloxane,fluorinated silicones and vinyl and hydride substituted silicones.

The nanocrystals used the present invention can be embedded in apolymeric (or other suitable material, e.g., waxes, oils) matrix usingany suitable method, for example, mixing the nanocrystals in a polymerand casting a film, mixing the nanocrystals with monomers andpolymerizing them together, mixing the nanocrystals in a so-gel to forman oxide, or any other method known to those skilled in the art. As usedherein, the term “embedded” is used to indicate that the nanocrystalsare enclosed within the polymer that makes up the majority component ofthe matrix.

The thickness of the layers of the present invention can be controlledby any method known in the art, such as spin coating and screenprinting. Such methods are especially useful when coating opticaldevices such as lenses or mirrors with the polymeric layers. While thevarious polymeric layers of the present invention can be any thicknessrequired, suitably, the layers will be less than about 100 mm inthickness, and down to on the order of less than about 1 mm inthickness. In other embodiments, the polymeric layers of the presentinvention can be on the order of 10's to 100's of microns in thickness.In one embodiment, the present invention provides nanocrystal dopedlayers that are greater than about 0.5 mm in thickness, and suitablywill scatter only a minimal portion of light that enters the layer (seelater for a discussion of scattering). In other embodiments, the layerswill be between about 0.5 mm and about 50 mm in thickness. In allembodiments of the present invention, the nanocrystals can be embeddedin the various matrixes at any loading ratio that is appropriate for thedesired function. Suitably, the nanocrystals will be loaded at a ratioof between about 0.001% and about 75% by volume depending upon theapplication, matrix and type of nanocrystals used. The appropriateloading ratios can readily be determined by the ordinarily skilledartisan and are described herein further with regard to specificapplications.

II. Photon-Filtering Nanocomposites

In another embodiment, the present invention provides polymeric layerscomprising a polymer and nanocrystals embedded within the polymer, suchthat the layers act as photon-filtering nanocomposites. Suitably, thenanocrystals will be prepared from semiconductor materials, but anysuitable material described throughout can be used to prepare thenanocrystals. In certain embodiments, the nanocrystals will have a sizeand a composition such that the nanocrystals absorb light of aparticular wavelength or over a range of wavelengths. As such, thenanocrystals utilized in these embodiments are tailored such that theirabsorption characteristics are enhanced or maximized, while theiremission characteristics are minimized, i.e. they will absorb light in ahighly efficient manner, but suitably will emit only a very low level,or preferably no light. In other embodiments, however, thephoton-filtering nanocomposites can also comprise nanocrystals that havehigh emission properties and emit light at a particular wavelength asdiscussed throughout. As such, the present invention providesnanocomposites that comprise different types of nanocrystals such thatthe nanocomposites exhibit several, or all, of the properties discussedthroughout, in a layer.

A photon-filtering nanocomposite in accordance with one embodiment ofthe present invention is shown in FIG. 6. FIG. 6 is a cross-sectionalview of a polymeric layer 600 showing nanocrystals 604 embedded inpolymer 602. Note that nanocrystals 604 are not to scale and are visiblyrepresented for illustrative purposes only. The polymeric layers andnanocomposites of the present invention can also comprise nanocrystalsof different sizes and compositions within the same layer.

In suitable embodiments, the nanocrystals can be distributedhomogenously throughout the polymeric layer and nanocomposites (see FIG.6). In other embodiments, the nanocrystals can be randomly distributed.In further embodiments, the nanocrystals can be distributed throughoutthe layer such that they form a nanocrystal density gradient throughoutthe layer (as discussed further in the refractive index section below).Such an embodiment is represented in FIG. 7, which shows across-sectional view of a polymeric layer 700 with nanocrystals 604embedded in polymer 602 in such a way that they form a nanocrystaldensity gradient from high density (lower portion of FIG. 7) to lowdensity (upper portion of FIG. 7) within polymer 602.

The photon-filtering polymeric layers and nanocomposites of the presentinvention can be used to coat, encapsulate, cover, be deposited on (orany other similar arrangement known to those skilled in the art) anysubstrate material. Suitably, the polymeric layers of the presentinvention can be used to coat optical devices. In other embodiment, thepolymeric layers can be used to encapsulate active devices.

In embodiments of the present invention where the photon-filteringpolymeric layers are used to coat optical devices, such optical devicescan be refractive (e.g., lenses) or reflective (e.g., mirrors). FIG. 8is a cross-sectional view of an optical device 802 coated with polymer602 comprising nanocrystals 604. Coated optical devices 800 according tosuch an embodiment can be used in any application where a filter oranti-reflective coating is desired on a refractive or reflective device.

In embodiments of the present invention where the photon-filteringpolymeric layers are used to encapsulate active devices, such activedevices can be any device known to the skilled artisan. As used hereinan “active device” is one that requires a source of energy for itsoperation and has an output that is a function of present and past inputsignals. Examples of active devices include, but are not limited to,controlled power supplies, transistors, diodes, including light emittingdiodes (LEDs), light detectors, amplifiers, transmitters and othersemiconductor devices.

By controlling the size and composition of the nanocrystals used in thepractice of the present invention, the nanocrystals will absorb light ofa particular wavelength, or a particular range of wavelengths, while notscattering light. The ability to make nanocrystals out of differentsemiconductors, and control their size, allows for polymeric layers tobe fabricated with nanocrystals that will absorb light from the UV, tovisible, to near infrared (NIR), to infrared (IR) wavelengths.Nanocrystals for use in the present invention will suitably be less thanabout 100 nm in size, and down to less than about 2 nm in size. Insuitable embodiments, the nanocrystals of the present invention absorbvisible light. As used herein, visible light is electromagneticradiation with wavelengths between about 380 and about 780 nanometersthat is visible to the human eye. Visible light can be separated intothe various colors of the spectrum, such as red, orange, yellow, green,blue, indigo and violet. The photon-filtering nanocomposites of thepresent invention can be constructed so as to absorb light that makes upany one or more of these colors. For example, the nanocomposites of thepresent invention can be constructed so as to absorb blue light, redlight, or green light, combinations of such colors, or any colors inbetween. As used herein, blue light comprises light between about 435 nmand about 500 nm, green light comprises light between about 520 nm and565 nm and red light comprises light between about 625 nm and about 740nm in wavelength. The ordinarily skilled artisan will be able toconstruct nanocomposites that can filter any combination of thesewavelengths, or wavelengths between these colors, and suchnanocomposites are embodied by the present invention.

Polymeric layers that comprise nanocrystals that can absorb light of aparticular wavelength, or range of wavelengths, will act as edge passfilters, absorbing light that is less than a certain wavelength. Forexample, the photon-filtering nanocomposites can be constructed so as toabsorb light that is less than about 565 nm (e.g., blue and green) andallowing wavelengths of light that are longer than about 565 nm (e.g.,red) to pass through the polymeric layer.

In other embodiments, the nanocrystals have a size and a compositionsuch that they absorb photons that are in the ultraviolet,near-infrared, and/or infrared spectra. As used herein, the ultravioletspectrum comprises light between about 100 nm to about 400 nm, thenear-infrared spectrum comprises light between about 750 nm to about 100μm in wavelength and the infrared spectrum comprises light between about750 nm to about 300 μm in wavelength.

While nanocrystals of any suitable material can be used in the practiceof the present invention, in certain embodiments, the nanocrystals canbe ZnS, InAs or CdSe nanocrystals. In one embodiment, InAs nanocrystals(with a 1340 nm absorption peak) with TOP (tri-n-octylphosphine) ligandsattached to their surface can be dissolved in a solvent such as toluene.Poly(vinyl butyral):poly(vinyl acetate) (PVB:PVA) polymer can also bedissolved in toluene and the two solutions can be mixed together. Asubstrate can then be coated or encapsulated with the mixture and thetoluene evaporated off. A thin film results that is non-light-scatteringdue to the size of the non-aggregated nanocrystals. Polymeric layersproduced in such a manner will have an effective refractive indexbetween that of either material by itself (i.e., the polymer or thenanocrystal material), which can be adjusted by modifying the loadingratio of the nanocrystals and the density of the nanocrystals at variouspoints in the polymeric layer (see Refractive Index section foradditional disclosure). A polymeric layer comprising such nanocrystalscan act as an antireflective filter, absorbing light that is less thanabout 1340 nm in wavelength.

In another embodiment of the present invention, CdSe nanocrystals(having an absorption peak at about 580 nm) with stearic acid ligandscan be dissolved in a solvent such as toluene. In other embodiments, ZnSnanocrystals with amine, carboxylic acid, phosphonic acid, phosphonate,phosphine, phosphine oxide or sulfur ligands can be dissolved in asolvent. In the case of CdSe nanocrystals, a ligand exchange can then beperformed in solution with a siloxane ligand and excess ligand can beremoved. Nanocrystals can then be mixed with a polymer base, such assilicone, and a substrate material can then be coated or encapsulated.After curing, the film will have an effective refractive index betweenthat of the polymer (e.g., silicone) and the nanocrystals, which can beadjusted by changing the loading ratio of the nanocrystals in thesilicone. Such a polymeric layer will act as a filter absorbing lightthat is less than about 580 nm in wavelength (i.e., blue, green, yellow,orange, violet, UV light).

III. Refractive Index Matching Nanocomposites

Poor extraction caused by total internal light reflection due to indexof refraction mismatches at interfaces is a problem for light emittingdevices, including LEDs. It is well known that light impinging aninterface between materials of index n and n′<n, at an angle θ relativeto the vertical will be totally reflected, if sin θ>sin θc=n′/n. For adirect extraction from GaN with n=2.26 into air n′=1, this limits theextraction cone within a solid angle ΔΩ=2π (1−cos θc), where θc is thecritical extraction angle, θc=26°, being just 10% of the totalextraction angle 2π (upper half only). In the encapsulation arrangementshown in FIG. 2, overall light extraction relates to: (a) extractionfrom substrate 202 to phosphor encapsulant layer 206; and (b) fromphosphor encapsulant layer 206 to air. The radius of the encapsulantlayer is much larger than the emitting area (e.g., cm vs. mm) thus lightrays inside the encapsulant layer that reach out in the radial directionimpinge almost perpendicular to the encapsulant layer surface θ<<θc andare extracted. Thus the overall extraction is mainly limited byextraction from the substrate to the phosphor plane interface, takingadvantage of the higher phosphor critical angle, due to the higher thanair index of refraction.

In one embodiment, as shown in FIG. 2, the present invention providesnanocomposite layers 208, that can bond to LED substrate 202, that havean effective refraction index of greater than 1.5. In suitableembodiments, by increasing the substrate effective refractive index to1.8, an angle of θc=68° is generated and the extraction efficiency isdoubled to 63%. In another embodiment, the present invention providesnanocomposites combining nanocrystals with a refractive index of about2.0 to about 3 with host matrix materials, including polymers (e.g.,TiO₂ with an effective refractive index 1.5 to 2, or silicone with arefractive index of about 1.49), to generate a nanocomposite materialwith an effective refractive index of about 2, with a criticalextraction angle of about θc=77°, thereby increasing the extractionefficiency to 78%. In further embodiments, a matched-index passive layer(e.g., a hard shell polymer) can be added above the phosphor layer totake advantage of the radial incidence, thereby enhancing extractionfrom the phosphor layer into the air.

As used herein, the term “effective refractive index (n_(e))” is used toindicate that the polymeric layers of the present invention arecomposite materials, and thus, their effective refractive index is aresult of the refractive index of all components of the layer. The termrefractive index as used herein indicates the degree to which thenanocomposites of the invention bend light. The refractive index of thenanocomposites can be determined by the ratio of the speed of light in avacuum divided by the speed of light in the nanocomposites.

The polymeric layers of the present invention, whether down-converting,photon-filtering, or refractive index matching, have an effectiverefractive index that can be controlled by the ratio, density,composition and size of the nanocrystals embedded within the matrix.FIG. 9 shows the effect of loading ratio of ZnS nanocrystals (n=2.35) onthe effective refractive index of various materials. For all matrixes,the effective refractive index increases linearly with the loading ratio(%) up to the refractive index of the pure ZnS nanocrystals. FIG. 10shows the effective refractive index of a silicone nanocompositecomprising 3 nm ZnS nanocrystals at 30% by volume as a function ofwavelength. An effective refractive index of greater than about 1.77 isobserved for all wavelengths from 300 nm to 700 nm.

Control and tailoring of the effective refractive index allows thematrixes of the present invention to be utilized in applications where alayer having either a uniform or varying effective refractive index maybe desired, for example as polymeric layers encapsulating LEDs. In suchapplications, a polymeric layer is used to encapsulate the lightemitting diode chip of an LED to provide protection to the chip. Asdiscussed above, due to refractive index (n) differences between thehigh n of the LED chip and the generally low n of the polymericencapsulant, large amounts of light are lost due to light reflecting atthe chip/polymer interface. The present invention therefore provides fora polymeric layer that has a refractive index higher than pure polymerthat can approach or match the refractive index of the LED chip, therebylimiting the light lost at the chip/polymer interface. Such anembodiment is represented in FIG. 11, showing a cross-sectional view ofan encapsulated light emitting diode 1100, in which polymer 602comprising embedded nanocrystals 604 encapsulates LED chip 1106. Anyactive device, including those discussed throughout, can be encapsulatedin a similar manner. Furthermore, FIGS. 11 and 12 showing specific LEDstructures are presented for illustrative purposes only, and any LEDstructure known to those skilled in the art can be similarlyencapsulated.

The effective refractive index of the polymeric layer can be any valuebetween that of the pure matrix material (e.g., silicone at about 1.49,TiO₂ at about 1.5) and the nanocrystals themselves (e.g., up to about3). Suitably, the effective refractive index of the matrix will begreater than about 1.5, preferably between about 1.5 and about 2.5, andin certain embodiments the refractive index of the matrix will be about1.8.

In other embodiments, in order to add further stability to an LEDstructure, a second polymeric layer can be added on top of the firstlayer. Often, this second layer will be a “hard-shell” polymer and willhave a refractive index that is lower than the LED chip. Thus, if therefractive index of the first polymeric layer is matched to therefractive index of the LED chip, reflections will occur at the firstpolymeric layer/hard shell polymer interface. In order to overcome thisproblem, in another embodiment, the present invention provides for apolymeric layer or polymeric encapsulant which has a density gradient ofnanocrystals such that the effective refractive index of the polymericlayer matches both the chip and the hard shell polymer at theirrespective interfaces.

In one such embodiment, the present invention provides polymeric layersthat encapsulate an active device that has an effective refractiveindex, n₁. The layer comprises a polymer and semiconductor nanocrystalsembedded within the polymer, and has an inner boundary in contact withthe active device and an outer boundary in contact with a medium thathas an effective refractive index, n₂. The layer has an effectiverefractive index less than or equal to n₁ at the inner boundary and aneffective refractive index greater than or equal to n₂ at the outerboundary. Suitably, the active device will be an LED, though any activedevice, including those described throughout, can be encapsulated. Insuitable embodiments, n₁ will be greater than n₂.

FIG. 12 shows a cross-sectional view of an LED encapsulated in such apolymeric layer. Encapsulated LED 1200 comprises polymeric layer 602comprising embedded nanocrystals 604 encapsulating LED chip 1106. Hardshell polymer 1202 further coats polymeric layer 602 to provideadditional structural integrity and protection to the LED. FIG. 12illustrates the nanocrystal density gradient throughout the thickness ofpolymeric layer 602, this gradient being highest at the boundary withLED chip 1106 and lowest at the boundary with hard shell polymer 1202.In such embodiments, the effective refractive index is n₁ at theboundary with LED chip 1106 and the effective refractive index is n₂ atthe boundary with hard shell polymer 1202. In certain embodiments, thisnanocrystal density gradient will be substantially linear throughout thepolymeric layer, though it can take any form throughout the thickness ofthe layer, e.g., cyclic, parabolic, etc. Suitably, the effectiverefractive index of polymeric layer 602 will be greater than about 1.5throughout the layer, and in certain embodiments will be about 1.8 (n₁)at the interface with LED chip 1106 and about 1.5 (n₂) at the interfacewith hard shell polymer 1202.

As shown in FIG. 13, light emitting diodes often utilize LED chip 1106covered by a drop or layer of silicone 1300 usually a few millimeters indiameter. As discussed throughout, by replacing the silicone cap in FIG.13 with nanocrystal doped matrixes with enhanced refractive indexes,more light can be extracted from LED chip 1106. However, two issues mayarise by doing so: (1) the amount of nanocrystals required for a dopedmatrix a few millimeters think for each LED translates to rather largequantities of nanocrystals for mass production, thereby driving up thecost; and (2) the scattering from the nanocrystals throughout a thinklayer may make the matrix opaque for a path-length of a few millimeters.

To resolve these issues, in another embodiment (see FIG. 14), thepresent invention provides for a thin film of nanocomposite 1402 formedon the surface of an LED chip 1106, this thin film is then furthercapped with small hemispheres 1404 of the same nanocomposite. All of thelight that enters the nanocomposite hits the composite/air interface at90° and therefore does not suffer from any internal reflection. Thethickness of the film and the diameter of the small caps can be chosento satisfy the thermal compliance and other mechanical/thermalrequirements. The thickness, t, of the film, and the diameter, d, of thesmall hemispheres, can be in the range of 10's-100's of nms to micronsto millimeters. Suitably, the thickness of the layer will be on theorder of 10's of microns, for example about 10-50 microns. The diameterof the hemispheres is generally on the order of microns.

In other embodiments of the present invention, the small hemispheres1404 of nanocomposite can be further capped with a large hemisphere ofsilicone 1302, as illustrated in FIG. 15. In this case, the refractiveindex of the large hemisphere of silicone is not required for lightextraction. The critical angle is only determined by the refractiveindex of the LED chip 1106, n1, and that of the nanocrystal doped matrix(1402 and 1404), n3 as:

$\theta_{critical} = {\sin^{- 1}{\frac{n_{1}}{n_{3}}.}}$

Preparation of nanocomposite films and hemispheres in this manner allowsfor the use of larger sized nanocrystals in comparison to those that canbe used in conjunction with thicker pathlength films. For example,nanocrystals on the order of 5-7 nm could be used with the thinfilm/hemisphere embodiments of the present invention, while nanocrystalson the order of about 3-5 nm could be required for thicker pathlengthnanocomposites.

As discussed throughout, the nanocrystals useful in the practice of thepresent invention can have a composition and a size such that theyabsorb light at a particular wavelength(s) and emit at a particularwavelength(s). In certain embodiments, the polymeric layers of thepresent invention can comprise combinations of nanocrystals thatfunction in the various ways described herein. For example, ananocomposite of the present invention can comprise nanocrystals havingspecific, enhanced emission properties, others having specific, enhancedabsorption properties but low emission properties, and the entirenanocomposite can be constructed such that the layer has a specificrefractive index that is matched or tailored for a specific purpose.Combined in such a way, the polymeric layers of the present inventioncan be used as encapsulants for active devices (e.g., LEDs) that emitlight of a certain wavelength, filter out other wavelengths and have arefractive index appropriately matched to an active device and/or anadditional substrate or coating.

IV. Size and Miscibility of Nanocrystals

In all embodiments of the present invention, it is desirable that thenanocrystals do not aggregate. That is, that they remain separate fromeach other in the polymeric layer and do not coalesce with one anotherto form larger aggregates. This is important, as individual crystalswill not scatter light passing through the layer, while largeraggregated structures can create an opaque layer that can hinder thepassage of light.

Suitably, whether functioning as down-converting layers,photon-filtering layers, refractive index matching layers, orcombinations thereof, the nanocomposites of the present invention willscatter a minimal portion of light that enters the various layers. It isdesirable that the nanocomposites of the present invention scatter aslittle light as possible, such that the layers are substantiallytransparent or clear.

As used herein, the phrase “scatter a minimal portion of light,” meansthat the amount of light that enters the various nanocomposites of theinvention from the incident side (the side that light is entering) istransmitted such that less than about 50% of this incident light isscattered by the nanocomposite. In suitable embodiments, the amount oflight that is scattered by the nanocomposite will be less than about20%, less than about 15%, and approaching 0% of the light beingtransmitted. The factors that impact most significantly on the amount oflight that is scattered by the nanocomposites are the size of thenanocrystals and their miscibility in the polymeric matrix, and hencetheir ability to remain separated. It should be understood that inapplications of the present invention where the nanocomposites functionas filters, the amount of light that is transmitted through thepolymeric layer will necessarily be reduced as certain wavelengths orranges of wavelengths will absorbed by the nanocrystals and filtered outof the incident light.

As discussed above, the size of the nanocrystals can be tailored byselecting specific semiconductor materials and then generating andprocessing the nanocrystals until the desired size is attained. In thevarious embodiments of the present invention, the nanocrystals willsuitably be between about 1 nm and about 20 nm in size. More suitably,between about 1 nm and about 10 nm, between about 1 nm and about 4 nmand most suitably between about 1 nm and about 3 nm. As shown in FIG.16, using a constant loading volume of ZnS nanocrystals (22% by volume)in silicone, the percent transmittance of light can be tailored frombetween about 5% to about 100% (i.e. percent that is scattered can betailored from between about 95% to about 0%). It is a significantadvantage of the present invention that by generating nanocrystals thatare between about 1 nm to about 4 nm, less than about 50% of theincident light is scattered by the nanocomposites of the presentinvention. As shown in FIG. 16, by creating nanocrystals that arebetween about 1 nm and about 3 nm, scattering of less than 20%,approaching 15%, 10%, 5% and 0%, can be achieved. As demonstrated inFIG. 17, a silicone nanocomposite, comprising 3 nm ZnS nanocrystals anda layer with a 3 mm pathlength will scatter less than about 50% (i.e.transmit more than about 50%) of the incident light over the wavelengthrange 350 nm to 700 mn, scatter less than about 30% over the wavelengthrange 400 nm to 700 nm and scatter less than about 15% over thewavelength range 500 nm to 700 nm.

Controlled Surface Chemistry for High Loading Density Nanocomposites

In the formation of the nanocomposites of the present invention, twocritical issues are: (1) achieving high miscibility of the nanocrystalsin the host matrix, and (2) prevention of aggregation of thenanocrystals at a high concentration. Aggregation results in quenchingof the emission, hence a lowering of the amount of light transmitted, aswell as light scattering from the aggregates. Tuning the index ofrefraction of the overall composite layer also occurs at differentnanocrystals loading densities. Since the nanocrystals have a refractiveindex of about 2.5 to about 3 and the host matrix is about 1.5 to about2, matching of the refractive index of the LED substrate (typicallysapphire or SiC) will eliminate an optical interface and losses fromtotal internal reflection.

As part of this approach, several issues are addressed, includingdetermination of whether the necessary loading densities in thenanocomposites as determined by simulations are achieved; whether thenanocrystals are homogenously embedded in the host matrix with no (orminimized) aggregation or phase separation so that a high quantum yieldis retained and scattering is prevented; whether the index of refractionof the composite layer can be tuned by adjusting the loading density(e.g., gradient) of nanocrystals in the host matrix; whether refractiveindices close to the LED substrate are achieved and what the projectedeffect on the light extraction efficiency is; and at the nanocrystalloading density for refractive index matching, what is the layerthickness of the composite layer necessary to reach an optical densityat the excitation wavelength to yield the optimized emission profiledetermined by the simulations. It can also be determined whether thisthickness is compatible with low cost, thin film processing (e.g.,thicknesses <1-2 microns).

In order to accomplish this, a tailored, miscibility-enhancing ligandcan be designed to bind, associate, coordinate, attach or conjugate to ananocrystal, and to allow for controlled mixing and miscibility in thehost matrix. The performance characteristics, including quantifying theeffects on the internal quantum efficiency and light extractionefficiency are measured on nanocomposites of various loading densitiesand thicknesses.

Surface Chemistry Modification

Dispersion of nanocrystals in a host matrix can be controlled byminimizing phase separation and aggregation that can occur when mixingthe nanocrystals into the matrixes. A basic strategy of the presentinvention is to design a novel 3-part ligand, in which the head-group,tail-group and middle/body-group can each be independently fabricatedand optimized for their particular function, and then combined into anideally functioning complete surface ligand (see FIG. 18; see FIG. 19for an example ligand). As shown in FIG. 18, head group 1804 is selectedto bind specifically to the semiconductor material of the nanocrystal(e.g., can be tailored and optimized for CdSe, ZnS or any othernanocrystal material). Tail group 1800 is designed to interact stronglywith the matrix material and be miscible in the solvent utilized (andcan, optionally, contain a linker group to the host matrix) to allowmaximum miscibility and loading density in the host matrix withoutnanocrystal aggregation. Middle or body group 1802 is selected forspecific electronic functionality (e.g., charge isolation).

This multipart ligand strategy has been used for the fabrication of highloading density, non-fluorescent, polymer-CdSe nanorod composites in thedevelopment of hybrid inorganic-organic nanocomposite solar cells. Incertain embodiments of the present invention, significant modificationsto the ligand are made due to differences in the two applications.Specifically, the ligand is designed to be charge insulating (ratherthan charge conducting) and to provide retention of nanocrystalphotoluminescence as well as to be compatible with a completelydifferent matrix type (inorganic rather than organic polymers) andnanocrystal material type and shape.

With the development of the 3-part ligand, control of the loadingdensity of the nanocrystals in the nanocomposite can be achieved forpurposes of creating the nanoparticle density gradients as described.This permits evaluation of the influence of quantum yield and opticalscattering in the nanocomposite. Additionally, tuning of the refractiveindex of the nanocomposite is possible since the index of refraction ofthe host matrix is known.

A benefit of this modular approach is the ability to rapidly evaluatenew tail, head, and middle/body groups. In the area of head groups(binding with the nanocrystal), there are available methods developedfor the development of CdSe synthetic techniques. This includes anunderstanding of the binding of nanocrystals with phosphonic acids,amines, thiols, phosphines, and phosphine oxides.

A tailored ligand can be optionally designed to bind strongly to thenanocrystal and to allow for mixing in a TiO₂ host medium. The newligand allows for dispersion control (solubility and processability) tofacilitate incorporation of the nanocrystals into solvents or hostmatrixes over a wide range of loading densities as necessary to achievethe optimal white light device performance characteristics andrefractive index matching to the blue-LED.

Ligand Synthesis

The ligand molecule can be synthesized using a generalized techniqueallowing three separate groups to be synthesized separately and thencombined. Head groups of phosphonic acid, amines, carboxylic acids orthiol moieties can be used because of their affinity for the nanocrystalsurface. Tail groups can contain terminal hydroxyl groups to tether thenanocrystal in a titania sol-gel matrix, or silicon groups to match asilicone polymer matrix. The middle/body unit is selected for chargeinsulation (e.g., large energy gap for both electrons and holes), andpossible targets are identified using computer modeling. The modeling isperformed using a Density Functional Theory (DFT) to model the bandgapof various target molecular structures for ligand design. Theconfirmation of the chemical identity and purity will be done using massspectrometry, NMR and FTIR analysis.

The insulating group (middle/body unit) of the ligands can be selectedfrom long-chain alkanes of various lengths and aromatic hydrocarbons,e.g., C6-C22 alkanes. Selection of the length of the body unit willdepend on the desired characteristics of the final matrix and of thepolymeric base being used. For example, in applications where it isdesired that the matrix possess rheologic or other properties (e.g.,mechanical/electical) similar to that of the polymeric base substance, ashorter chain (e.g., C6-C18) body unit can be selected. For example, theuse of a C12 body unit-based ligand on ZnS nanocrystals allows forincreased loading of the ZnS nanocrystals at a ratio sufficient toachieve a refractive index of 1.7070 in a base of immersion oil(starting refractive index 1.5180), while still maintaining the greaselike consistency of the oil. The use of shorter chain ligands allows fora lower volume fraction of nanocrystals to be used to achieve the samerefractive index when compared to nanocrystals with longer chainligands.

In other applications, the use of longer chain ligands (e.g., C18-C22)can be used when it is desired that the final matrix possess propertiescloser to that of the ligand itself, rather than the base material. Incertain applications, the ligand itself could be used to form the matrixmaterial. Longer chain ligands also allow for additional spacing betweenthe nanocrystals to keep them from aggregating in the base substrate.

FIG. 19 shows an example of a ligand with a phosphonic acid head group1900, an aliphatic tail 1902 and an aromatic hydrocarbon body 1904.Appropriate choice of body and/or tail components are used to providelike functionality to the matrix to afford high concentrations ofnanocrystals in siloxane polymer matrices. Refractive index (RI)tuning/matching can also be affected by the ligand. Independent tuningof the tail or body components of the ligand to obtain a particular RImatch with the polymer matrix can be achieved by varying the ligandchemistry appropriately.

The general design “Head-Body-Tail” affords freedom from any particularensemble limitations. For example: a phosphonate head group fornanocrystal binding, alkane body group for length adjustment/nanocrystalspacing and dimethyl silicone tail for silicone matrix compatibility canbe synthesized as shown in FIG. 20 b. An example of tuning the RI(increasing the value) can be realized by incorporation of phenyl groupsshown in FIG. 20 a (similar to silicone polymers (from vendor GelestInc., 612 William Leigh Drive Tullytown, Pa. 19007-6308): DMS-H21dimethylsiloxane vs. HPM-502 phenyl-methylsiloxane, 1.403 and 1.500refractive index values, respectively) in the siloxane tail. FIG. 20 aillustrates several non-limiting example ligands with head-body-taildesign. Matrix compatibility adjustments such as branching siloxaneunits can also be accommodated (FIG. 20 b, molecule 3). Structureverification by NMR of synthesized precursors 1 and 2 in FIG. 20 b isshown in FIGS. 20 c-f.

FIG. 20 g shows additional examples of ligands and synthesis schemes inaccordance with the present invention, including (from top to bottom)the use of trisiloxane, cyclic tetrasiloxane and branched siloxane tailgroups in the generation of ligands. Carboxylic acid functionalizationof these silicone surfactants is illustrated in FIG. 20 g. Structureverification by NMR of a carboxylated trisiloxane ligand shown in FIG.20 g (ligand at top of page) is represented in FIGS. 20 h-i.

FIGS. 20 j and 20 n show further examples and synthesis schemes for theproduction of phosphonate functionalized silicone ligands. Structureverification by NMR of the bromide precursor shown in FIG. 20 j isrepresented in FIG. 20 k. FIGS. 20 l and 20 m represent NMR verificationof the structure of the phosphonate ligand product.

Ligand exchange to displace the surfactants, which are used duringsynthesis, can be done by mass action exchange in solution. Thenanocrystals and the new ligand are co-dissolved in solvent and allowedto react for a specified time at elevated temperatures. The product isprecipitated with alcohol to remove any excess unbound ligands and toremove the displaced synthesis surfactants. The attachment is confirmedby NMR analysis by redissolving the product into a deuterated NMRcompatible solvent. Complexation of the ligand with the nanocrystalcauses a shift and broadening in the spectrum compared to the freeunbound molecule due to a hindered rotation.

In another embodiment, the present invention provides polymeric layers,comprising a polymer; and semiconductor nanocrystals embedded within thepolymer, wherein the nanocrystals have miscibility-enhancing ligandsconjugated to their surface, and wherein the ligands comprise an alkanechain of between 6 and 18 carbons in length. In suitable embodiments,the ligands can comprise an alkane chain of between 12 and 18 carbons inlength. The polymer will suitably be silicone, and the semiconductornanocrystals will suitably have a size between about 1-10 nm, and incertain embodiments will be ZnS nanocrystals. In certain embodiments,the polymeric layers will scatter a minimal portion of light that enterssaid polymeric layer. Suitably, the layer will be greater than about 0.5mm in thickness.

V. Processes for Producing Nanocomposites

In another embodiment, as represented in FIG. 21, the present inventionprovides processes for preparing polymeric layers, comprising (a) mixingsemiconductor nanocrystals at a first density with a solvent and apolymer to form a first mixture (2100), (b) coating a substrate materialwith the first mixture (2102), and (c) evaporating the solvent to formthe polymeric layer (2104), wherein the polymeric layer has an effectiverefractive index of n₁.

In suitable embodiments, the processes of the present invention can beused to coat active devices or optical devices. As discussed throughout,nanocrystals useful in the processes of the present invention cancomprise miscibility-enhancing ligands conjugated, coordinated,attached, bound or otherwise associated to their surface. Any of thevarious types of nanocrystals discussed herein can be used in theprocesses of the present invention. For example, high emissionnanocrystals, low emission/high absorption nanocrystals and lowemission/low absorption nanocrystals can be used. In certainembodiments, two or more different types of nanocrystals can be mixedwith the solvent and polymer, thereby creating a composite that hasseveral or all of the properties described herein. Refractive indexmatching applications can utilize any of the nanocrystals discussedthroughout, depending on if the nanocomposite is also required tofunction as a down-converting layer or a filtering layer. In otherapplications, nanocrystals that have low emission/low absorptionproperties are useful in refractive index matching applications whererefractive index effects only are desired.

In other embodiments, as shown in FIG. 21, the processes of the presentinvention can further comprise (d) mixing semiconductor nanocrystals ata second density with a solvent and a polymer to form a second mixture(2106), (e) coating the substrate material with the second mixture(2108), and (f) evaporating the solvent to form a second polymeric layer(2110), wherein the second polymeric layer has an effective refractiveindex of n₂.

In other embodiments, the processes of the present invention can furthercomprise repeating steps (d) through (f) with a third through i^(th)density of semiconductor nanocrystals to produce third through i^(th)polymeric layers, wherein the third through i^(th) polymeric layers haveeffective refractive indices, n₃ through n_(i), respectively (2112). Asused herein, “i” refers to an integer. The present invention encompassesprocesses for producing polymeric layers which comprise any number ofseparate layers used to produce an overall layer, coating, orencapsulant. Each individual layer, 1 through i, can comprise adifferent density of nanocrystals, nanocrystals of a differentcomposition (i.e., high emission or high absorptive properties), andnanocrystals of different sizes. As such, each layer can have adifferent effective refractive index and can have multiple and/ordifferent properties and characteristics.

By providing individual polymeric layers each with a potentiallydifferent effective refractive index, an overall polymeric layer (e.g.,an encapsulating layer) can be generated that has a nanocrystal densitygradient throughout the overall layer, and also an effective refractiveindex gradient throughout the overall layer. FIG. 22 illustrates thatthe effective refractive index of the 1^(st) layer, n₁ (2200), will begreater than any other layer (2202, 2204, 2206), and the effectiverefractive index of the i^(th) layer, n_(i) (2206), will be less thanany other layer (2200, 2202, 2204). It should also be noted that theprocesses of the present invention can be performed in the reverseorder, i.e., where the nanocrystal density and thus the effectiverefractive index of the i^(th) layer is higher than any other layer, andthe effective refractive index of the first layer prepared, n₁, is lessthan any other layer. In other embodiments, the density and effectiverefractive index of the individual layers can be the same, or can beprepared in such a manner that the overall effective refractive index ofthe polymeric layer varies throughout the layer, rather than in a gradedfashion, as in FIG. 22.

As discussed throughout, various known processes can be used to coat asubstrate material with the polymeric layers of the present invention,as would become apparent to people having ordinary skill in the art andbased on the description herein. Suitable coating processes include, butare not limited to, spin coating and screen printing.

In general, spin coating consists of four stages. The first stage is thedeposition of the coating fluid onto the substrate. It can be done usinga nozzle that pours the coating solution out, or can be sprayed onto thesurface, etc. Usually this dispense stage provides a substantial excessof coating solution compared to the amount that will ultimately berequired in the final coating thickness. The second stage is when thesubstrate is accelerated up to its final, desired, rotation speed. Thesecond stage is usually characterized by aggressive fluid expulsion fromthe substrate surface by the rotational motion. Ultimately, thesubstrate reaches its desired speed and the fluid is thin enough thatthe viscous shear drag exactly balances the rotational accelerations.The third stage is when the substrate is spinning at a constant rate andfluid viscous forces dominate fluid thinning behavior. This stage ischaracterized by gradual fluid thinning. Mathematical treatments of theflow behavior show that if the liquid exhibits Newtonian viscosity(i.e., is linear) and if the fluid thickness is initially uniform acrossthe substrate (albeit rather thick), then the fluid thickness profile atany following time will also be uniform, leading to a uniform finalcoating. The fourth stage is when the substrate is spinning at aconstant rate and solvent evaporation dominates the coating thinningbehavior. As the prior stage advances, the fluid thickness reaches apoint where the viscosity effects yield only rather minor net fluidflow. At this point, the evaporation of any volatile solvent specieswill become the dominant process occurring in the coating.

In another embodiment, the processes of the present invention canfurther comprise centrifuging the mixture produced in step 2100 to forma nanocrystal density gradient within the mixture prior to the coatingin 2102. The use of centrifugation creates a gradient within thepolymeric layer as nanocrystals separate in accordance with theirinertia. Various centrifugation speeds or accelerations can be used toproduce the nanocrystal density gradient in the polymeric layers and canreadily be determined by those skilled in the art. The centrifugationspeed selected depends on the size of the nanocrystals and thedifference in density between the nanocrystals and the polymer solutionprior to polymerization, and the centrifugal approach. Centrifugationcan be for a short time at high speed and generate a gradientkinetically where the centrifugation step is timed based on a calculatedor measured centrifugation rate. Alternatively, an equilibrium approachcan be used where the flux of the nanocrystals toward the bottom of acentrifuge tube is matched to the flux of nanocrystals toward the top ofthe tube (due to diffusion). The diffusional flux is proportional to theconcentration gradient of the nanocrystals. Suitably, accelerations canbe in the range of a few hundred times g to 100,000 times g, where g isthe acceleration due to gravity (9.8 m/s²) By selecting nanocrystals ofdifferent sizes and made from different materials, the nanocrystals willspread out through the polymeric layer according to their inertia inresponse to the centrifugation and generate a gradient in the layer. Anyother process known to those skilled in the art to generate gradientswithin polymers may also be used to create the polymeric layers of thepresent invention.

In optical lenses, the optical path length varies with distance from itscenter, where optical path length is defined as the product of thephysical path length, thickness, and the refractive index, n, of thelens material. In the most common lenses, the refractive index, n, isfixed and the thickness, varies. However, a lens can also be created bykeeping the thickness, constant and varying the refractive index as afunction of distance from the axis of the lens. Such a lens is called aGraded Index lens, or sometimes abbreviated as a GRIN lens. The methodsof the present invention can also be used to create GRIN lenses.Polymer/nanocrystal blends can be used to make GRIN lenses due to thedramatic refractive index difference between nanocrystals (e.g., ZnSabout 2.35) and optical plastics such as poly(methyl methacrylate)(PMMA) (refractive index about 1.45). With normal glass, a difference ofabout 0.05 refractive index units is achievable over about 8 mm.Utilizing the methods and processes of the present application, adifference of about 0.20 refractive index units over about 8 mm can beachieved to make much more powerful lenses.

In such embodiments, a gradient pump can be used to inject a solutioncontaining polymer monomers and nanocrystals into the center of a mold,and then nanocrystal concentration can be varied during the fill. Thelens can then be cured and removed.

The polymeric nanocomposites of the present invention can be used in anyapplication where the down-conversion, filtering, and/or refractiveindex characteristics of the composites are desired. Non-limitingexamples of applications of polymeric nanocomposites with increasedrefractive indexes include:

Super High Gloss Coatings: Increasing the refractive index of atransparent coating increases gloss. The addition of nanocrystals (e.g.,ZnS nanocrystals) to polymeric coatings such as waxes and other coatings(e.g., car waxes, shoe waxes, floor coatings and related products) wouldincrease the amount of light that is reflected from the coated surfaceand thus increase the glossiness of its appearance. Appropriate ligands,including C18, PEG and others discussed throughout could be used so asto allow the nanocrystals to be formulated with various polymers, waxesand coatings.

Plastic Eye Glass Lenses and Contacts: The thickness of a lens isproportional to the refractive index of the material of which it made.The higher the refractive index, the thinner the lens. Normal glass hasa refractive index of about 1.523 while an example plastic, such asCR39, has refractive index of 1.49. A plastic lens, although lighter inweight, is thicker than a glass lens of equivalent power.

By incorporating nanocrystals, for example ZnS nanocrystals suitablywith the appropriate ligands, into a plastic lens, the refractive indexcan be increased beyond the level of glass to make ultra-thin lenses. Inapplications such as contact lenses, there is an even more pressing needto create thin lenses due to the importance of oxygen transport throughthe lens to the eye. The refractive index of contact lenses are about1.40. The addition of even a small percentage of nanocrystals (e.g.,about 10% ZnS) would increase the refractive index to about 1.5,therefore allowing for thinner lenses. Ligands such as those discussedthroughout can be used to lock the nanocrystals in place in thepolymeric layer. The addition of nanocrystals with specific absorptiveproperties, e.g., ultraviolet (UV) absorbing nanocrystals, would allowfor the creation of UV (or other wavelength) blocking lenses.

EXAMPLES

The following examples are illustrative, but not limiting, of the methodand compositions of the present invention. Other suitable modificationsand adaptations of the variety of conditions and parameters normallyencountered in nanocrystal synthesis, and which would become apparent tothose skilled in the art, are within the spirit and scope of theinvention.

Example 1

Core/shell Nanocrystal Synthesis

Suitable nanocrystal synthesis procedures include fabricatingnanocrystal samples with specific spectral characteristics matched tothose prescribed by the theoretical models of the present invention.This can include fabricating nanocrystals with tunable sizes and sizedistributions (e.g., sizes ranging from 1-20 nm in diameter producingemission peak wavelengths tunable between 460 and 640 nm with FWHMtunable from about 15 to about 100 nm). This in turn is used tosynthesize nanocrystal mixtures identified by simulations that have theoptimal emission characteristics. The simulation and core/shellnanocrystal procedure is typically performed in an iterative process.

Type I core-shell nanocrystals of CdSe/ZnS (core/shell) can besynthesized by a two step process using a solution phase method, firstwith the fabrication of the core material followed by growth of theshell.

Core Synthesis

Stock solutions are prepared of Se powder dissolved intri-n-butylphosphine (TBP), and Cd(CH₃)₂ dissolved in TBP. In anair-free environment, the Cd stock solution is added drop-wise to amixture of trioctylphosphine oxide (TOPO), and trioctylphosphine (TOP),which was previously degassed at 120° C. The temperature is raised to300° C., followed by a quick injection of the Se precursor. Afterinjection, the temperature drops to around 260° C., which is heldconstant for a period of time to control the size of the particle. Bycontrolling the temperature profile and starting reagents andconditions, the center-wavelength and size-distribution can be tunedindependently. The identity of the product is confirmed using XRD andTEM analysis.

Shell Synthesis

Core CdSe nanocrystals are dispersed in TOPO and TOP to which a mixtureof ZnEt₂ and (TMS)₂S will be added at a temperature between 140° C. to220° C. ZnS shell coating thickness will be varied by changing precursorratios and growth temperatures to obtain a uniform surface coverage andto improve the quantum efficiency. The confirmation of shell growth willbe done using XRD, EDX and TEM analysis.

The optical properties of the individual nanocrystals are characterizedby measurement of the UV-Vis absorption and photoluminescence spectrausing a commercial UV-Vis spectrophotometer and a fluorometer. Theexcitation wavelength is matched to the blue LED (about 460 nm).Internal quantum efficiency of the nanocrystals in solution arecalculated using internal reference standards. Nanocrystal componentmixtures (solution phase) are formed by mixing the appropriateconcentration ratios to match the predictions from the theoreticalmodel. The emission and absorption information of these actual mixturesis then back-fed as an input into the simulation to validate (and torefine, if necessary) the model.

The output of this procedure is a solution-phase mixture of nanocrystalsthat has the appropriate composition to produce white light with CRI andCTT matching that of the theoretical model when illuminated with blueexcitation and total down-conversion efficiency comparable to thatpredicted by the model, assuming zero loss to other mechanisms in theprocess.

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. It will be apparent to persons skilledin the relevant art that various changes in form and detail can be madetherein without departing from the spirit and scope of the invention.Thus, the breadth and scope of the present invention should not belimited by any of the above-described exemplary embodiments, but shouldbe defined only in accordance with the following claims and theirequivalents.

Example 2

ZnS Nanocrystal Synthesis

In the order listed, add the following to a 50 mL 3-neck round bottomflask:

1. Zn(acetate)₂: 76.5 mg Lot# 12727BC

2. Stearic Acid: 484 mg Lot# 06615MA

3. Tri-n-octylphosphine oxide (TOPO): 4.07 g Lot# 21604LA

In a glove box prepare the following:

3.9 g of distilled tri-n-octylphosphine (TOP) (#35-111) in 5 mL syringe;

116.4 mg of stock solution 02-190 (bis(trimethylsilyl)sulfide(TMS₂S):TOP) in 1 mL syringe; and

One 40 mL septa cap vial with 5.0 mL of MeOH

Place reactor under vacuum

Heat to 120° C.

Once at 120° C., allow to sit for 20 minutes

Place reactor under argon

Slowly inject TOP from 5 mL syringe

Change set point temperature to 250° C.

Once at 250° C., immediately inject the stock solution 02-190(bis(trimethylsilyl)sulfide (TMS₂S):TOP) from 1 mL syringe

Grow with temperature at 250° C. for 2 minutes

Remove the heating mantle and allow reaction to cool to 50° C.

At 50° C., use a syringe to remove the growth solution and inject itinto the 40 mL vial with MeOH.

FIG. 23 shows an X-Ray diffraction scan of ZnS nanocrystals producedaccording the present invention. The scan shows the presence of ZincSulfide with a mixture of wurtzite and zinc blend (spaelite) crystals.

FIG. 24 shows a Transmission Electron Micrograph (TEM) of ZnSnanocrystals (about 4 nm diameter) produced according the presentinvention.

The ZnS nanocrystals can be produced using any chain length hydrocarbon,for example C6-C22 alkane, depending on the application and desiredproperties.

Example 3

Carboxylic Acid-Silicone Ligand Synthesis

General Methods

All manipulations were carried out with strict exclusion of air andmoisture by using Schlenk technique under an atmosphere of dry nitrogen,unless otherwise stated. THF, toluene, chloroform-d₁ and toluene-d₈ weredried over activated 4 A Molecular Sieves and de-gassed by threefreeze-pump-thaw cycles. 4-pentenoic acid and1,1,1,3,5,5,5-heptamethyltrisiloxane were purchased from Aldrich (St.Louis, Mo.), distilled and stored in a storage flask using Schlenktechnique before use. Heptamethyl cyclotetrasiloxane and1,1,1,3,3,5,5-heptamethyl trisiloxane were purchased from Gelest(Morrisville, Pa.), distilled and stored in a storage flask usingSchlenk technique before use. Karstedt's catalyst or platinum divinyltetramethyl disiloxane complex, 2.1 to 2.4% in xylenes, was purchasedfrom Gelest, stored in the glove box and used without furtherpurification. All products were stored in the glove box. NMR chemicalshift data were recorded with a Bruker FT NMR at 400 MHz for ¹H, 100 MHzfor ¹³C{¹H}, 162 MHz for ³¹P{¹H} and 79.5 MHz for ²⁹Si{¹H} and arelisted in ppm.

General Synthesis Procedure (See FIG. 20 g)

Synthesis of HO₂C(CH₂)₄(SiMe₂O)₂SiMe₃

In a glove box, the following reaction was set up in a 100 mL Schlenkflask by addition of Karstedt's catalyst (2.66 g solution, 0.300 mmol)followed by dilution in THF, 60 mL, on the Schlenk line. Then to theclear colorless solution, 1,1,1,3,3,5,5-heptamethyltrisiloxane (8.13 mL,6.67 g, 30.0 mmol) was added by syringe over about 90 seconds and inabout 30 seconds turned the solution clear green. The solution wasstirred at room temperature for about 15 minutes. Then, with thereaction flask surrounded by a room temperature water bath, 4-pentenoicacid (3.07 mL, 3.00 g, 30.0 mmol) was added by syringe over about 90seconds which slowly turned the solution light brown and produced asmall amount of heat. After about 2 hours, the water bath was heated to35° C. using a heater controlled by a thermostat and stirred overnight.

The volatiles were removed from the clear brown solution by rotationalevaporator leaving an opaque brown oil. The product was distilled fromthe mixture by short path apparatus collecting the fraction with vaportemperature between 80 and 95° C. and pressure<20 mtorr. The product isa clear colorless oil (5.55 g or approximately 17 mmol and 57% yield)that typically contains about 50% acid and 50% anhydride. Completeconversion to the acid was accomplished by dissolution of the productmixture (2.00 g or approximately 6.2 mmol) in acetonitrile, 40 mL,followed by addition of pyridine (5.00 mL, 5.11 g, 64.6 mmol) and water(6.20 mL, 6.20 g, 33.4 mmol). The solution was stirred overnight in air.The volatiles were removed from the solution by rotational evaporatoruntil the residue was reduced to an oil. Next, toluene, 100 mL, wasadded and the volatiles removed by rotational evaporator until theresidue was reduced to an oil. The water removal using toluene azeotropewas performed twice. The resulting clear colorless oil was transferredinto a beaker producing a layer about 3 mm thick and the product driedin a desiccator over phosphorous pentoxide under static vacuum of <10mtorr overnight. The product was a clear colorless oil (1.35 g, 4.18mmol, 67% yield) and was stored in the glove box.

Additional carboxylic acid-silicone ligands such as those shown in FIG.20 g and disclosed throughout the present specification, can be preparedusing a procedure similar to that above.

Analysis of HO₂C(CH₂)₄(SiMe₂O)₂SiMe₃ ¹H NMR (chloroform-d₁, δ): 0.10,0.13, 0.14 (s, SiMe), 0.52, 1.39, 1.67 (m, CH2), 2.35 (t, 2H, CH2).¹³C{¹H} NMR (chloroform-d₁, δ): 1.5, 2.0, 2.0 (s, SiMe), 18.1, 23.1,28.5, 34.1 (s, CH2), 180.5 (s, C═O). ²⁹Si{¹H} (1:1 CDCl₃/Et₃N, 0.02 MCr(acac)₃, δ): −20.9, 7.1 (s, 1:2). IR (cm⁻¹, diamond): 1050 s(Si—O—Si), 1700 m (C═O), 3030 w (CH aromatic), 2956 sh, 2928 s, 2854 m(CH aliphatic), 3400 to 2700 v br (acid). Mass Spec ESI (m/z): 345(MNa+).

Data for Synthesis and Analysis of HO₂C(CH₂)₄SiMeO(SiMe₂)₃ (cyclictetrasiloxane)

The boiling point of the anhydride/acid mixture was 95 to 110° C. at apressure of <10 mbar. The yield for synthesis of the acid/anhydridemixture was about 64% and the conversion to acid was 63%.

¹H NMR (chloroform-d₁, δ): 0.10, 0.12, 0.13 (s, SiMe), 0.48, 1.39, 1.65(m, 2H, CH₂), 2.35 (t, 2H, CH₂). ¹³C{¹H} NMR (chloroform-d₁, δ): −0.1,1.9, 2.0 (s, SiMe), 17.5, 22.9, 28.3, 34.1 (s, CH₂), 180.4 (s, C═O).²⁹Si{¹H} (1:1 CDCl₃/Et₃N, 0.02 M Cr(acac)₃, δ): −20.3, −19.1, −19.0 (s,1:2:1). IR (cm⁻¹, diamond): 1050 s (Si—O—Si), 1700 m (C═O), 3030 w (CHaromatic), 2956 sh, 2928 s, 2854 m (CH aliphatic), 3400 to 2700 v br(acid).

Data for Synthesis and Analysis of HO₂C(CH₂)₄SiMe(OSiMe₃)₂

The boiling point of the anhydride/acid mixture was 78 to 95° C. at apressure of <10 mbar. The yield for synthesis of the acid/anhydridemixture was 63% and the conversion to acid was 62%.

¹H NMR (chloroform-d₁, δ): 0.10, 0.12, 0.13 (s, SiMe), 0.53, 1.43, 1.68(m, 2H, CH₂), 2.35 (t, 2H, CH₂). ¹³C {¹H} NMR (chloroform-d₁, δ): 0.9,1.0 (s, SiMe), 16.9, 22.7, 28.1, 34.0 (s, CH2), 180.0 (s, C═O). ²⁹Si{¹H}(1:1 CDCl₃/Et₃N, 0.02 M Cr(acac)₃, δ): −22.0, −7.1 (s, 1:2). IR (cm⁻¹,diamond): 1050 s (Si—O—Si), 1700 m (C═O), 3030 w (CH aromatic), 2956 sh,2928 s, 2854 m (CH aliphatic), 3400 to 2700 v br (acid). Mass Spec ESITOF 381 (MH+) and ESI TOF 379 (M−H).

Example 4

Phosphonic Acid—Silicone Ligand Synthesis

General Synthesis Procedure

Synthesis of (EtO)₂P(O)(CH₂)₄(SiMe₂O)₂SiMe₃

In a glove box, Karstedt's catalyst (0.450 g solution, 0.052 mmol) wasadded to a 250 mL Schlenk flask. On the Schlenk line, THF, 100 mL, wasadded and followed by 1,1,1,3,5,5,5-heptamethyl trisiloxane (14.0 mL,11.5 g, 51.8 mmol) by syringe over about 90 seconds. The clear colorlesssolution turned clear green in about 30 seconds. The reaction solutionwas stirred for about 15 minutes before addition of diethyl 3-butenylphosphonate (10.0 mL, 9.95 g, 51.8 mmol) by syringe over about 90seconds. The reaction solution then slowly turned light brown andproduced a small amount of heat. After about 2 hours, the reaction flaskwas surrounded by a thermostat controlled water bath that was heated to35° C. The reaction solution was heated overnight.

The volatiles were removed from the clear brown solution by rotationalevaporator leaving an opaque brown oil. A column was packed with silica(230-400 mesh) in hexanes that was 30 mm in diameter and 150 mm long.After placing the crude product on the column, the column was elutedwith hexane, 250 mL, followed by a mixed solvent of 1:1 ratio of ethylacetate to hexane, 1500 mL. The elutant was collected in one fraction.Next the volatiles were then removed by rotational evaporator leaving alight brown oil. The product was then distilled using a simpledistillation at pressure of <20 mtorr and pot temperature of 120° C. Theproduct was a clear colorless oil (17.6 g, 42.5 mmol, 82.1% yield).

Additional phosphonic acid-silicone ligands such as those shown in FIGS.20 a, 20 j and 20 n and disclosed throughout the present specificationcan be prepared using a procedure similar to that above.

Analysis of (EtO)₂P(O)(CH₂)₄(SiMe₂O)₂SiMe₃ ¹H NMR (chloroform-d₁, δ):0.00 (s, 6H, SiMe), 0.05 (s, 6H, SiMe), 0.07 (s, 9H, SiMe), 0.53, 1.39,1.60, 1.70 (m, 2H, CH₂), 1.30 (t, 6H, CH₂CH₃), 4.06 (m, 4H, CH₂CH₃). ¹³C{¹H} NMR (chloroform-d₁, δ): −0.34, 1.46, 2.01 (s, SiMe), 16.68, 61.52(d, JP—C=6 Hz, CH₂CH₃O), 18.05 (s, CH₂), 24.62 (d, JP—C=18 Hz, CH₂),26.19 (d, JP—C=5 Hz, CH₂), 25.69 (d, JP—C=140 Hz, CH₂P). ³¹P {¹H} NMR(chloroform-d₁, δ): 32. ²⁹Si{¹H} (1:1 CDCl₃, 0.02 M Cr(acac)₃, δ):−22.00, 7.12 (s, 1:2). IR (cm⁻¹, diamond): 1030 (s, Si—O—Si), 1260 (m,Si—Me), 1380, 1400 1430 (w, Et-O—P).

Data for Synthesis and Analysis of (EtO)₂P(O)(CH₂)₄SiMe(OSiMe₃)₂

The pot was heated to 120° C. at a pressure of <20 mtorr to distill theproduct as a clear colorless oil in 81% yield.

¹H NMR (chloroform-d₁, δ): −0.32 (s, 3H, SiMe), 0.06 (s, 18H, SiMe),0.44, 1.37, 1.60, 1.70 (m, 2H, CH₂), 1.30 (t, 6H, CH₂CH₃), 4.08 (m, 4H,CH₂CH₃). ¹³C {¹H} NMR (chloroform-d₁, δ): −0.15, 2.01 (s, SiMe), 16.65,61.49 (d, JP—C=6 Hz, CH₂CH₃O), 17.38 (s, CH₂), 24.48 (d, JP—C=18 Hz,CH₂), 25.97 (d, JP—C=5 Hz, CH₂), 25.71 (d, JP—C=140 Hz, CH₂P). ³¹P {¹H}NMR (chloroform-d₁, δ): 33. ²⁹Si{¹H} (1:1 CDCl₃, 0.02 M Cr(acac)₃, δ):−17.96, 9.94, 10.00 (s, 1:1:1). IR (cm⁻¹, diamond): 1030 (s, Si—O—Si),1250 (m, Si—Me), 1380, 1400, 1430 (w, Et-O—P).

Data for Synthesis and Analysis of (EtO)₂P(O)(CH₂)₄SiMeO(SiMe₂)₃ (CyclicTetrasiloxane)

For the distillation the vapor temperature was 84 to 96° C. at apressure of <10 mtorr. The product was isolated as a clear colorless oilin 44% yield.

¹H NMR (chloroform-d₁, δ): 0.50, 0.70 (s, 21H total, SiMe), 0.51 1.41,1.61, 1.69 (m, 2H each, CH₂), 1.30 (t, 6H, CH₂CH₃), 4.08 (m, 4H,CH₂CH₃). ¹³C {¹H} NMR (chloroform-d₁, δ): −0.57, 0.91, 0.94 (s, SiMe),16.66, 61.50 (d, JP—C=6 Hz, CH₂CH₃O), 16.86 (s, CH₂), 24.29 (d, JP—C=18Hz, CH₂), 25.88 (d, JP—C=5 Hz, CH₂), 25.70 (d, JP—C=140 Hz, CH₂P). ³¹P{¹H} NMR (chloroform-d₁, δ): 33. ²⁹Si{1H} (1:1 CDCl₃, 0.02 M Cr(acac)₃,δ): −20.39, −19.17, −19.08 (s, 1:2:1). IR (cm⁻¹, diamond): 1015, 1050(s, Si—O—Si), 1250 (m, Si—Me), 1380, 1400, 1430 (w, Et-O—P).

General Synthesis Procedure for the Phosphonic Acid,(HO)₂P(O)(CH₂)₄(SiMe₂O)₂SiMe₃

In a 50 mL Schlenk flask, CH₂Cl₂, 15 mL, was added followed by(EtO)₂P(O)(CH₂)₄(SiMe₂O)₂SiMe₃ (1.00 g, 2.42 mmol) and the solutionstirred until homogenous. Then trimethylsilyl bromide (0.671 mL, 0.778g, 5.08 mmol) was added and the solution was stirred for 15 minutes.

The volatiles were removed by vacuum transfer and 10.0 mL of methanolwas added followed by 0.25 mL of water. After stirring for 30 minutes,the volatiles were removed by vacuum transfer and 10.0 mL of toluene wasadded and the solution was stirred for 1 minute. The volatiles wereremoved by vacuum transfer and 10 ml of toluene was added, the solutionstirred and the volatiles removed again, as before. The product was aslightly cloudy viscous oil.

Analysis of (HO)₂P(O)(CH₂)₄(SiMe₂O)₂SiMe₃ ESI (m/z): 359 (MH+) and 381(MNa+).

All publications, patents and patent applications mentioned in thisspecification are indicative of the level of skill of those skilled inthe art to which this invention pertains, and are herein incorporated byreference to the same extent as if each individual publication, patentor patent application was specifically and individually indicated to beincorporated by reference.

1. A polymeric layer in combination with an optical device, comprising:(a) a polymer; (b) semiconductor nanocrystals; (c) a ligand compositionattached to the nanocrystals wherein the ligand composition is differentfrom the polymer, wherein the nanocrystals with ligand compositionattached thereto are mechanically embedded within said polymer, theligand composition comprising a body structure comprising an alkanechain of 4 carbons or between 6 and 22 carbons in length, a head groupchemically coupled to the nanocrystals and to the body structure at afirst position on the alkane chain and a tail group chemically coupledto the body structure at a second position on the alkane chain, whereinthe tail group comprises one or more silicon groups or one or moreterminal hydroxyl groups; and (d) an optical device, wherein saidpolymeric layer encapsulates the optical device and scatters less thanabout 50% of incident light that enters said layer from said opticaldevice.
 2. The polymeric layer of claim 1, wherein said nanocrystalshave a size and a composition such that said nanocrystals absorbultraviolet light.
 3. The polymeric layer of claim 1, wherein saidnanocrystals have a size and a composition such that said nanocrystalsabsorb near-infrared light.
 4. The polymeric layer of claim 1, whereinsaid nanocrystals have a size and a composition such that saidnanocrystals absorb infrared light.
 5. The polymeric layer of claim 1,wherein said optical device is a refractive lens or a reflectiveelement.
 6. The polymeric layer of claim 1, wherein said optical deviceis a light emitting diode.
 7. The polymeric layer as in claim 1, whereinsaid polymer is silicone.
 8. The polymeric layer as in claim 1, whereina size of said nanocrystals is between about 1-10 nm.
 9. The polymericlayer of claim 8, wherein said size of said nanocrystals is betweenabout 1-4 nm.
 10. The polymeric layer of claim 8, wherein said size ofsaid nanocrystals is between about 1-3 nm.
 11. The polymeric layer as inclaim 1, wherein said polymeric layer has an effective refractive indexof about 1.5.
 12. The polymeric layer of claim 11, wherein saidpolymeric layer has an effective refractive index of about 1.8.
 13. Thepolymeric layer as in claim 1, wherein said layer is greater than about0.5 mm in thickness.
 14. The polymeric layer of claim 1, wherein saidnanocrystals absorb blue light.
 15. The polymeric layer of claim 1,wherein said nanocrystals absorb green light.
 16. The polymeric layer ofclaim 1, wherein said nanocrystals absorb red light.
 17. A polymericlayer in combination with an active device, wherein the polymeric layerencapsulates the active device, said active device having an effectiverefractive index, n1, said layer comprising: (a) a polymer; (b)semiconductor nanocrystals; (c) a ligand composition attached to thenanocrystals wherein the ligand composition is different from thepolymer, wherein the nanocrystals with the ligand composition attachedthereto are mechanically embedded within said polymer, the ligandcomposition comprising a body structure comprising an alkane chain of 4carbons or between 6 and 22 carbons in length, a head group chemicallycoupled to the nanocrystals and to the body structure at a firstposition on the alkane chain and a tail group chemically coupled to thebody structure at a second position on the alkane chain, wherein thetail group comprises one or more silicon groups or one or more terminalhydroxyl groups; and (d) an active device, wherein said polymeric layerhaving an inner boundary in contact with said active device and an outerboundary in contact with a medium having an effective refractive index,n2, wherein said polymeric layer has an effective refractive index lessthan or equal to n1 at said inner boundary and an effective refractiveindex greater than or equal to n2 at said outer boundary.
 18. Thepolymeric layer of claim 17, wherein n1 is greater than n2.
 19. Thepolymeric layer of claim 17, wherein said layer has a nanocrystaldensity gradient, said gradient being highest at said inner boundary andlowest at said outer boundary.
 20. The polymeric layer of claim 19,wherein said nanocrystal density gradient is substantially linearthroughout said layer.
 21. The polymeric layer of claim 17, wherein saidactive device is a light emitting diode.
 22. The polymeric layer ofclaim 17, wherein said nanocrystals have a size between about 1-3 nm.23. A polymeric layer in combination with an optical device, comprising:(a) a polymer; (b) semiconductor nanocrystals; (c) miscibility-enhancingligands conjugated to a surface of the nanocrystals wherein the ligandsare different from the polymer, wherein the nanocrystals with ligandsconjugated thereto are mechanically embedded within said polymer andwherein said ligands comprise a body structure comprising an alkanechain of between 18 and 22 carbons in length, a head group chemicallycoupled to the nanocrystals and to the body structure at a firstposition on the alkane chain and a tail group chemically coupled to thebody structure at a second position on the alkane chain, and (d) anoptical device, wherein said polymeric layer encapsulates the opticaldevice and scatters less than about 50% of incident light that enterssaid layer from said optical device.
 24. The polymeric layer of claim23, wherein said semiconductor nanocrystals have a size between about1-3 nm.
 25. The polymeric layer of claim 23, wherein said layer scattersless than about 20% of light that enters said polymeric layer.
 26. Thepolymeric layer of claim 23, wherein said layer scatters less than about15% of light that enters said polymeric layer.